Fatty Acid Synthesis

1
Fatty Acid Synthesis

• Lecture 16
• Modified from internet sources, journals and books

2
Fatty Acid Synthesis

• Prediction the pathway for the synthesis of
  fatty acids would be the reversal of the
  oxidation pathway
• this would not allow distinct regulation of the
  two pathways to occur even given the fact that
  the pathways are separated within different
  cellular compartments
• pathway for fatty acid synthesis occurs in the
  cytoplasm (oxidation occurs in the mitochondria)
• the essential chemistry of the two processes ?
  reversals of each other

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• oxidation and synthesis of fats utilize an
  activated two carbon intermediate ? acetyl-CoA
• acetyl-CoA in fat synthesis ? exists temporarily
  bound to the enzyme complex as malonyl-CoA
• synthesis of malonyl-CoA ? the first committed
  step of fatty acid synthesis
• the enzyme that catalyzes this reaction ?
  acetyl-CoA carboxylase (ACC) the major site of
  regulation of fatty acid synthesis

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The rate of fatty acid synthesis

• controlled by the equilibrium between monomeric
  ACC and polymeric ACC
• activity of ACC requires polymerization ? the
  conformational change is enhanced by citrate and
  inhibited by long-chain fatty acids
• ACC is also controlled through hormone mediated
  phosphorylation (see below).
• The acetyl-CoA and malonyl-CoA are transferred to
  ACP (acetyl-CoA phosphatase) by the action of
  acetyl-CoA transacylase and malonyl-CoA
  transacylase, respectively

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continued

• attachment of these carbon atoms to ACP allows
  them to enter the fatty acid synthesis cycle.
• The synthesis of fatty acids from acetyl-CoA and
  malonyl-CoA ? carried out by fatty acid synthase
  (FAS)

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continued

• All of the reactions of fatty acid synthesis are
  carried out by the multiple enzymatic activities
  of FAS (fatty acid synthase)
• like fat oxidation ? fat synthesis involves 4
  enzymatic activities
• ß-keto-ACP synthase, ß-keto-ACP reductase, 3-OH
  acyl-ACP dehydratase and enoyl-CoA reductase (the
  two reduction reactions require NADPH oxidation
  to NADP)
• the primary fatty acid synthesized by FAS is
  palmitate then released from the enzyme and can
  then undergo separate elongation and/or
  unsaturation to yield other fatty acid molecules

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Origin of Cytoplasmic Acetyl-CoA

• Acetyl-CoA ? generated in the mitochondria
  primarily from two sources
• the pyruvate dehydrogenase (PDH) reaction
• fatty acid oxidation
• these acetyl units to be utilized for fatty acid
  synthesis ? they must be present in the cytoplasm
• shift from fatty acid oxidation and glycolytic
  oxidation occurs when the need for energy
  diminishes
• This results in ? reduced oxidation of acetyl-CoA
  in the TCA cycle and the oxidative
  phosphorylation pathway
• Under these conditions ? the mitochondrial acetyl
  units can be stored as fat for future energy
  demands

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• Acetyl-CoA ? enters the cytoplasm in the form of
  citrate via the tricarboxylate transport system
• In the cytoplasm ? citrate is converted to
  oxaloacetate and acetyl-CoA (by the ATP driven
  ATP-citrate lyase reaction)
• resultant oxaloacetate ? is converted to malate
  by malate dehydrogenase (MDH)

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• The malate produced by this pathway ? can undergo
  oxidative decarboxylation by malic enzyme
• co-enzyme for this reaction is NADP generating
  NADPH
• advantage of this series of reactions for
  converting mitochondrial acetyl-CoA into
  cytoplasmic acetyl-CoA ? the NADPH produced by
  the malic enzyme reaction can be a major source
  of reducing co-factor for the fatty acid synthase
  activities

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Regulation of Fatty Acid Metabolism

• must consider the global organismal energy
  requirements in order to effectively understand
  how the synthesis and degradation of fats (and
  also carbohydrates) needs to be exquisitely
  regulated
• blood ? is the carrier of triacylglycerols in the
  form of VLDLs and chylomicrons, fatty acids bound
  to albumin, amino acids, lactate, ketone bodies
  and glucose
• The pancreas ? is the primary organ involved in
  sensing the organisms dietary and energetic
  states via glucose concentrations in the blood

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• The regulation of fat metabolism occurs via
  distinct mechanisms
• short term regulation ? regulation effected by
  events such as substrate availability, allosteric
  effectors and/or enzyme modification
• ACC (acetyl-CoA carboxylase) ? the rate-limiting
  (committed) step in fatty acid synthesis

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• two major isoforms of ACC in mammalian tissues
• ACC1 and ACC2
• ACC1 ? is strictly cytosolic and is enriched in
  liver, adipose tissue and lactating mammary
  tissue
• ACC2 ? originally discovered in rat heart but is
  also expressed in liver and skeletal muscle
• Both isoforms of ACC ? allosterically activated
  by citrate and inhibited by palmitoyl-CoA and
  other short- and long-chain fatty acyl-CoAs

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• Citrate ? triggers the polymerization of ACC1
  which leads to significant increases in its
  activity
• ACC2 ? does not undergo significant
  polymerization (presumably due to its
  mitochondrial association), is allosterically
  activated by citrate
• Glutamate and other dicarboxylic acids can also
  allosterically activate both ACC isoforms

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• ACC activity can also be affected by
  phosphorylation
• Glucagon-stimulation ? increases in cAMP and
  subsequently increasing PKA activity also lead to
  phosphorylation of ACC and ACC2
• This insulin-mediated effect ? has not been
  observed in hepatocytes or adipose tissues cells
• Activation of a-adrenergic receptors in liver and
  skeletal muscle cells ? inhibits ACC activity as
  a result of phosphorylation (undetermined kinase)

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• Control of a given pathways' regulatory enzymes
  can also occur by alteration of enzyme synthesis
  and turn-over rates ? these changes are long term
  regulatory effects
• Insulin ? stimulates ACC and FAS synthesis,
  whereas, starvation leads to decreased synthesis
  of these enzymes
• Adipose tissue lipoprotein lipase levels ? also
  are increased by insulin and decreased by
  starvation

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• in contrast to the effects of insulin and
  starvation on adipose tissue ? their effects on
  heart lipoprotein lipase are just the inverse
• this allows the heart to absorb any available
  fatty acids in the blood in order to oxidize them
  for energy production
• Adipose tissue ? contains hormone-sensitive
  lipase (HSL), that is activated by PKA-dependent
  phosphorylation leading to increased fatty acid
  release to the blood

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• In the liver ? the net result of activation of
  HSL (due to increased acetyl-CoA levels) is the
  production of ketone bodies
• This would occur under conditions where
  insufficient carbohydrate stores and
  gluconeogenic precursors were available in liver
  for increased glucose production
• Insulin ? has the opposite effect to glucagon and
  epi leading to increased glycogen and
  triacylglyceride synthesis
• One of the many effects of insulin ? to lower
  cAMP levels which leads to increased
  dephosphorylation through the enhanced activity
  of protein phosphatases

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ChREBP Master Lipid Regulator in the Liver

• ChREBP helix-loop-helix/leucine zipper
  (bHLH/LZ) transcription factor,
  carbohydrate-responsive element-binding protein ?
  has emerged as a central regulator of lipid
  synthesis in liver
• ChREBP ? identified as a major glucose-responsive
  transcription factor and it is required for
  glucose-induced expression of the hepatic isozyme
  of the glycolytic enzyme pyruvate kinase
  (identified as L-PK)
• ChREBP ? acts to induce lipogenic genes such as
  acetyl-CoA carboxylase (ACC) and fatty acid
  synthase (FAS)

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• Expression of the ChREBP gene ? induced in the
  liver in response to increased glucose uptake
• Under conditions of low (basal) glucose
  concentration ? ChREBP is phosphorylated and
  resides in the cytosol
•  An emerging model of the role of ChREBP in
  overall glucose and lipid metabolism ? indicates
  it a master regulator of glucose-mediated lipid
  homeostasis not only in the liver but also in
  adipose tissue

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Elongation and Desaturation

• The fatty acid product released from FAS is
  palmitate (a 160 fatty acid, i.e. 16 carbons and
  no sites of unsaturation)
• Elongation and unsaturation of fatty acids ?
  occurs in both the mitochondria and endoplasmic
  reticulum
• The predominant site of these processes ? the ER
  membranes
• Elongation ? involves condensation of acyl-CoA
  groups with malonyl-CoA
• resultant product ? two carbons longer (CO2 is
  released from malonyl-CoA as in the FAS reaction)
  which undergoes reduction, dehydration and
  reduction yielding a saturated fatty acid
• Mitochondrial elongation ? involves acetyl-CoA
  units and is a reversal of oxidation

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• Desaturation occurs in the ER membranes
• involves 4 broad specificity fatty acyl-CoA
  desaturases (non-heme iron containing enzymes)
• These enzymes ? introduce unsaturation at C4, C5,
  C6 or C9
• electrons transferred from the oxidized fatty
  acids during desaturation ? are transferred from
  the desaturases to cytochrome b5 and then
  NADH-cytochrome b5 reductase
• These electrons ? are un-coupled from
  mitochondrial oxidative-phosphorylation and do
  not yield ATP

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• Since these enzymes cannot introduce sites of
  unsaturation beyond C9 ? they cannot synthesize
  either linoleate (182?9,12) or linolenate
  (183?9,12,15)
• These fatty acids must be acquired from the diet
  ? referred to as essential fatty acids
• Linoleic ? especially important in that it is
  required for the synthesis of arachidonic acid
• arachindonate ? a precursor for the eicosanoids
  (the prostaglandins and thromboxanes)

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continued

• role of fatty acids in eicosanoid synthesis ?
  that leads to poor growth, wound healing and
  dermatitis in persons on fat free diets
• linoleic acid ? a constituent of epidermal cell
  sphingolipids that function as the skins water
  permeability barrier

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Synthesis of Triglycerides

• Fatty acids ? stored for future use as
  triacylglycerols in all cells, but primarily in
  adipocytes of adipose tissue
• fatty acids present in triacylglycerols ?
  predominantly saturated
• major building block for the synthesis of
  triacylglycerols, in tissues other than adipose
  tissue, glycerol
• Adipocytes lack glycerol kinase ?
  dihydroxyacetone phosphate (DHAP), produced
  during glycolysis, is the precursor for
  triacylglycerol synthesis in adipose tissue
• adipoctes must have glucose to oxidize in order
  to store fatty acids in the form of
  triacylglycerols

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• The glycerol backbone of triacylglycerols ?
  activated by phosphorylation at the C-3 position
  by glycerol kinase
• The fatty acids incorporated into
  triacylglycerols ? activated to acyl-CoAs through
  the action of acyl-CoA synthetases
• Two molecules of acyl-CoA ? esterified to
  glycerol-3-phosphate to yield 1,2-diacylglycerol
  phosphate (commonly identified as phosphatidic
  acid).

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• The phosphate is then removed ? to yield
  1,2-diacylglycerol, the substrate for addition of
  the third fatty acid
• Intestinal monoacylglycerols, derived from the
  hydrolysis of dietary fats, can also serve as
  substrates for the synthesis of
  1,2-diacylglycerols

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Phospholipid Structures

• Phospholipids ? synthesized by esterification of
  an alcohol to the phosphate of phosphatidic acid
  (1,2-diacylglycerol 3-phosphate)
• Most phospholipids ? a saturated fatty acid on
  C-1 and an unsaturated fatty acid on C-2 of the
  glycerol backbone
• The most commonly added alcohols serine,
  ethanolamine and choline
• The major classifications of phospholipids are

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Phosphatidylcholine (PC)
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PC

• This class of phospholipids ? also called the
  lecithins
• At physiological pH ? phosphatidylcholines are
  neutral
• contain primarily palmitic or stearic acid at
  carbon 1 and primarily oleic, linoleic or
  linolenic acid at carbon 2
• lecithin dipalmitoyllecithin ? a component of
  lung or pulmonary surfactant
• the major (80) phospholipid found in the
  extracellular lipid layer lining the pulmonary
  alveoli

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Phosphatidylethanolamine (PE)
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PE

• These molecules are neutral at physiological pH
• contain primarily palmitic or stearic acid on
  carbon 1 and a long chain unsaturated fatty acid
  (e.g. 182, 204 and 226) on carbon 2

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Phosphatidylserine (PS)
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PS

• composed of fatty acids similar to the
  phosphatidyl-ethanol-amines
• PE is in the lipid bilayer of the a membrane

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Phosphatidylinositol (PI)
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PI

• contain almost exclusively stearic acid at carbon
  1 and arachidonic acid at carbon 2
• molecules exist in membranes with various levels
  of phosphate esterified to the hydroxyls of the
  inositol
• Molecules with phosphorylated inositol ?
  polyphosphoinositides
• polyphosphoinositides ? important intracellular
  transducers of signals emanating from the plasma
  membrane

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• One polyphosphoinositide (phosphatidylinositol
  4,5-bisphosphate, PIP2) ? a critically important
  membrane phospholipid involved in the
  transmission of signals for cell growth and
  differentiation from outside the cell to inside

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Phosphatidylglycerol (PG)
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PG

• Phosphatidylglycerols ? found in high
  concentration in mitochondrial membranes and as
  components of pulmonary surfactant
• Phosphatidylglycerol ? a precursor for the
  synthesis of cardiolipin (important component of
  the inner mitochondrial membrane, where it
  constitutes about 20 of the total lipid)
• vital role of PG ? serve as the precursor for the
  synthesis of diphosphatidylglycerols (DPGs)

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Diphosphatidylglycerol (DPG)
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DPG

• These molecules ? very acidic
• primarily in the inner mitochondrial membrane and
  also as components of pulmonary surfactant

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• The fatty acid distribution at the C-1 and C-2
  positions of glycerol within phospholipids is
  continually in flux
• phospholipid degradation and the continuous
  phospholipid remodeling that occurs while these
  molecules are in membranes ( highly dynamic
  systems)
• Phospholipid degradation ? results from the
  action of phospholipases
• various phospholipases exhibiting substrate
  specificities for different positions in
  phospholipids
• remodeling of acyl groups in phospholipids the
  result of the action of phospholipase A1 (PLA1)
  and phospholipase A2 (PLA2)

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Sites of Action of the Phospholipases A1, A2, C
and D.
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continued

• products of these phospholipases ? called
  lysophospholipids and can be substrates for acyl
  transferases utilizing different acyl-CoA groups
• PLA2 ? an important enzyme, whose activity is
  responsible for the release of arachidonic acid
  from the C-2 position of membrane phospholipids
• released arachidonate ? a substrate for the
  synthesis of the eicosanoids
• there is not just a single PLA2 enzyme At least
  19 enzymes have been identified with PLA2
  activity ? involved in numerous processes
  including modification of eicosanoid generation,
  host defense, and inflammation

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• The cytosolic PLA2 family (cPLA2) ? essential
  component of the initiation of arachidonic acid
  metabolism
• the sPLA2 enzymes ? tightly regulated by Ca2 and
  by phosphorylation

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Plasmalogens

• Plasmalogens are glycerol ether phospholipids
• Three major classes of plasmalogens have been
  identified
• choline, ethanolamine and serine plasmalogens
• Ethanolamine plasmalogen ? prevalent in myelin
• Choline plasmalogen ? abundant in cardiac tissue.
• One choline (1-O-1'-enyl-2-acetyl-sn-glycero-3-ph
  osphocholine) ? identified as an extremely
  powerful biological mediator ? is called platelet
  activating factor PAF

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• PAF functions as
• a mediator of hypersensitivity, acute
  inflammatory reactions and anaphylactic shock
• PAF is synthesized in response to the formation
  of antigen-IgE complexes on the surfaces of
  basophils, neutrophils, eosinophils, macrophages
  and monocytes
• synthesis and release of PAF from cells ? leads
  to platelet aggregation and the release of
  serotonin from platelets
• PAF also produces responses in liver, heart,
  smooth muscle, and uterine and lung tissues

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Platelet activating factor
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Metabolism of the Sphingolipids

• The sphingolipids (like the phospholipids) ?
  composed of a polar head group and two nonpolar
  tails
• core of sphingolipids ? the long-chain amino
  alcohol, sphingosine

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Sphingosine
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Basic composition of a ceramide"n" indicates any
fatty acid may be N-acetylated at this position
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• The sphingolipids ? include the sphingomyelins
  and glycosphingolipids (the cerebrosides,
  sulfatides, globosides and gangliosides)
• Sphingolipids ? a component of all membranes but
  are particularly abundant in the myelin sheath
• Sphingomyelins are sphingolipids
• Sphingomyelins ? important structural lipid
  components of nerve cell membranes

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A Sphingomyelin
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• Defects in the enzyme acid sphingomyelinase ?
  result in the lysosomal storage disease known as
  Niemann-Pick disease
• NP disease ? caused by acid sphingomyelinase
  deficiencies
• due to defects in the NPC1 gene and a NPC2 gene
• four principal classes of glycosphingolipids are
• cerebrosides, sulfatides, globosides and
  gangliosides

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• Cerebrosides ? most common of these is galactose
  (galactocerebrosides)
• Galactocerebrosides ? found predominantly in
  neuronal cell membranes
• glucocerebrosides ? not normally found in
  membranes they represent intermediates in the
  synthesis or degradation of more complex
  glycosphingolipids
• Excess lysosomal accumulation of
  glucocerebrosides is observed in Gaucher disease

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A Glucocerebroside
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Clinical Significances of Sphingolipids

• Some of the most devastating inborn errors in
  metabolism ? those associated with defects in the
  enzymes responsible for the lysosomal degradation
  of membrane glycosphingolipids (particularly
  abundant in the membranes of neural cells)
• Many of these disorders ? lead to severe
  psycho-motor retardation and early lethality
• the disorders are caused by defective lysosomal
  enzymes ? result being lysosomal accumulation of
  pathway intermediates
• these diseases ? often referred to as lysosomal
  storage diseases

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Pathways and intermediates in glycosphingolipid
metabolism

• Enzymes are indicated in green and the disease(s)
  associated with defects in the indicated enzyme
  are shown in blue
• SAP-A, SAP-B, SAP-C, and SAP-D the saposins
  which are a family of small glycoproteins
• The saposins (A, B, C, and D) are all derived
  from a single precursor ? prosaposin
• mature saposins, as well as prosaposin ? activate
  several lysosomal hydrolases involved in the
  metabolism of various sphingolipids

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Disorders Associated with Abnormal Sphingolipid
Metabolism

• Tay-Sachs disease
• infantile form rapidly progressing mental
  retardation, blindness, early mortality
• Gaucher disease
• hepatosplenomegaly, mental retardation in
  infantile form, long bone degeneration

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• Fabry disease
• kidney failure, skin rashes
• Niemann-Pick diseases
• type A is severe disorder with heptosplenomegaly,
  severe neurological involvement leading to early
  death, type B only visceral involvement

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Clinically important classes of sphingolipids

• One of the most clinically important classes of
  sphingolipids ? those that confer antigenic
  determinants on the surfaces of cells ?
  particularly the erythrocytes
• ABO blood group antigens ? the carbohydrate
  moieties of glycolipids on the surface of cells
  as well as the carbohydrate portion of serum
  glycoproteins
• When present on the surface of cells? the ABO
  carbohydrates are linked to sphingolipid and are
  therefore of the glycosphingolipid class

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• When the ABO carbohydrates are associated with
  protein in the form of glycoproteins ? are found
  in the serum and are referred to as the secreted
  forms
• Some individuals produce the glycoprotein forms
  of the ABO antigens while others do not
• This property distinguishes secretors from
  non-secretors, a property that has forensic
  importance such as in cases of rape.

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RDS

• A significant cause of death in premature infants
  and, on occasion, in full term infants
  respiratory distress syndrome (RDS) or hyaline
  membrane disease
• caused by an insufficient amount of pulmonary
  surfactant
• normal conditions ? the surfactant is synthesized
  by type II endothelial cells and is secreted into
  the alveolar spaces to prevent atelectasis
• Surfactant ? comprised primarily of
  dipalmitoyllecithin (additional lipid components
  include phosphatidylglycerol and
  phosphatidylinositol)

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• During the third trimester ? the fetal lung
  synthesizes primarily sphingomyelin, and type II
  endothelial cells convert the majority of their
  stored glycogen to fatty acids and then to
  dipalmitoyllecithin
• Fetal lung maturity ? can be determined by
  measuring the ratio of lecithin to sphingomyelin
  (L/S ratio) in the amniotic fluid
• An L/S ratio less than 2.0 indicates a potential
  risk of RDS
• The risk is nearly 75-80 when the L/S ratio is
  1.5

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• The carbohydrate portion of the ganglioside, GM1,
  present on the surface of intestinal epithelial
  cells ? the site of attachment of cholera toxin,
  the protein secreted by Vibrio cholerae
• These are just a few examples of how
  sphingolipids and glycosphingolipids are involved
  in various recognition functions at the surface
  of cells