Page 609 - Hematology_ Basic Principles and Practice ( PDFDrive )

518 Part V Red Blood Cells

the catabolism of branched-chain amino acids, odd-chain fatty acids, Substitutions
and cholesterol). When formed, succinyl-CoA can then enter the N position only N position only
10
5
Krebs tricarboxylic acid cycle.
In the cytoplasm, cobalamin, as methylcobalamin, functions as Formyl–CHO Formyl–CHO
a coenzyme for methionine synthase, a critical enzyme for which Methyl–CH 3 Hydroxymethyl–CH OH
2
both folates and cobalamin are required for normal one-carbon Formimino–CH=NH
5
metabolism (see Fig. 39.3). Methionine synthase is a modular protein Bridged between N and N 10
with four distinct and separate regions for binding homocysteine, Methylene–CH -
5-methyl-tetrahydrofolate (5-methyl-THF; 5-methyl-H 4PteGlu), the Methenyl–CH= 2 COOH
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cobalamin prosthetic group, and S-adenosylmethionine (SAM). The C H
reaction proceeds by methyl transfer from 5-methyl-tetrahydrofolate OH H N CO NH CH
to methionine synthase–bound cob(I)alamin to form methylcobala- N 9 CH 2 10 CH
min, followed by transfer of this methyl group to homocysteine to N 3 4 5 6 H 2
form methionine and regeneration of cob(I)alamin. In this process, 2 1 8 7 p-aminobenzoic CH 2 COOH
5-methyl-tetrahydrofolate is converted to tetrahydrofolate 5-methyl- NH 2 N N acid residue CO NH CH
tetrahydrofolate is converted to tetrahydrofolate that is subsequently H H (PABA)
polyglutamylated by folyl polyglutamate synthase; this addition of L-Glutamic CH 2
multiple glutamic acid moieties to tetrahydrofolate facilitates both its Substituted pteridine acid COOH
retention within cells and participation in one-carbon metabolism. moiety residue CH 2
During this reaction, spontaneous oxidation of cob(I)alamin (which CO NH CH
has no axial ligand) to the catalytically inactive cob(II)alamin form (tetrahydro)pteroyl moiety CH
requires reduction back to cob(I)alamin before it can accept a methyl 2
group. There is a specific redox regulator known as methionine CH 2
synthase reductase that restores enzyme activity in the presence of 5, 6, 7, 8 tetrahydropteroyl diglutamic acid
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SAM and NADPH ; this enzyme is mutated in patients with cblE COOH
mutations. Multiple glutamic
The physiologic importance of the key cofactor roles of the two acid moieties can
forms of cobalamin (i.e., adenosylcobalamin and methylcobalamin) be added
in methylmalonyl-CoA mutase and methionine synthase, respectively,
is that the products and by-products of these enzymatic reactions are Fig. 39.4 FOLATE CHEMISTRY AND NOMENCLATURE. Folic acid
critical for DNA, RNA, and protein biosynthesis. (pteroylmonoglutamate [PteGlu]) is the commercially available parent com-
pound for more than 100 compounds collectively referred to as folates. PteGlu
consists of three basic components: a pteridine derivative, a p-aminobenzoic
FOLATES acid residue, and an L-glutamic acid residue. Before PteGlu can play a role
as a coenzyme, it must first be reduced at positions 7 and 8 to dihydrofolic
Nutrition acid (H 2PteGlu) and then to 5,6,7,8-tetrahydrofolic acid (THF; H 4PteGlu),
and one to six additional glutamic acid residues must then be added by means
of γ-peptide bonds to the L-glutamate moiety (for which the subscripted n
Folates (the anionic forms of folic acid, also called vitamin B 9 ) are
synthesized by microorganisms and plants, including leafy vegetables in PteGlu n denotes polyglutamation). Folate coenzymes donate or accept
(spinach, lettuce, broccoli), beans, fruits (bananas, melons, lemons), one-carbon units in numerous reactions in amino acid and nucleotide
15
yeast, and mushrooms, and are also found in animal meats ; see Fig. metabolism. The various substitutions in H 4PteGlu n occur at positions 5 or
39.4 for chemistry and nomenclature. 10, or both; position 5 can be substituted by methyl (CH 3 ), formyl (CHO),
Among natural folates, which are predominantly in polyglu- or formimino (CHNH), and position 10 can be substituted by formyl or
tamylated form, only one-half are bioavailable; by contrast, 85% hydroxymethyl (CH 2OH). Positions 5 and 10 can be bridged by methylene
of folic acid that is added to food or ingested as a supplement (–CH 2–) or methenyl (–CH=). For an engaging account of the history of
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is bioavailable. Several factors can influence the bioavailability of folic acid, see the article by Hoffbrand and Weir.
folates. These include: (1) The stability of the food folate. Natural
reduced folates are labile and susceptible to oxidative cleavage by
nitrates or light exposure, but folic acid is much more stable. Pro- Absorption
longed boiling or cooking over 30 minutes reduces natural folates
by 50% to 80%, whereas ascorbate increases bioavailability, and After dietary folate polyglutamates are converted to folate monogluta-
refrigeration of leafy foods exposed to fluorescent light in supermar- mates at the enterocyte brush border, they are transported through the
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kets can double the folate content. (2) Pureed foods allow easier duodenal and jejunal brush border by physiologically relevant, high-
access to the glutamate carboxypeptidase II (also known as folate- affinity membrane-associated, luminal surface–facing PCFT, which
polyglutamate hydrolase), which converts folate polyglutamates to are most efficient in an acidic milieu. At pH 5.5, there is equivalent
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simpler folate monoglutamates before absorption ; any perturba- affinity for transport of physiologic reduced folates and folic acid,
tion of this enzyme by organic acids (orange juice), sulfasalazine, but at pH 6.5, reduced 5-methyl-tetrahydrofolate is transported more
32
or ethanol can preclude absorption; conversely, folate-binding efficiently. PCFT is a folate-hydrogen symporter, so with each folate
proteins in human or cow’s milk can increase folate absorption molecule transported, there is a net translocation of positive charge.
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for infants and women. (3) Interference with folate absorption Loss-of-function mutations in PCFT in the enterocyte and choroid
across the proximal jejunum from intestinal diseases will affect plexus result in (congenital) hereditary folate malabsorption, 33,34 a
the bioavailability of food folate. (4) Drugs that interfere with the condition associated with an inability to transport folate across the
proton-coupled folate transporter (PCFT) will compromise folate intestine and the choroid plexus. The expression of PCFT is increased
absorption. in folate-deficient mice, suggesting a physiologic regulatory mecha-
The recommended daily allowance of folate is as follows: adult nism. Proton pump inhibitors can reduce expression, and blocking
men and nonpregnant women, 400 µg; pregnant women, 600 µg; the function of PCFT by sulfasalazine and pyrimethamine can lead
lactating women, 500 µg; children 9 to 18 years, between 300 and to acquired folate malabsorption. 34,35 Within the enterocyte, folates
10
400 µg. A balanced Western diet contains adequate amounts of are reduced to tetrahydrofolate and methylated before release into
folate, but the net dietary intake of folate in many developing coun- plasma as 5-methyl-tetrahydrofolate. Most of the folic acid taken up
tries is often insufficient to sustain folate balance. 15,29–31 by the PCFT in the proximal small intestine is also converted within