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522 Part V Red Blood Cells

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accelerated by heavy chain ferritin, but the clinical significance is still Methylation
unclear.
After methionine is generated, it can be converted to a methyl donor
Compartmentalization and Channeling of through its adenosylation to SAM. SAM is a universal donor of
methyl groups for critically important biologic methylation reactions
Folate Metabolism involving over 80 proteins, membrane phospholipids, the synthesis
of neurotransmitters, RNA, DNA, and histones. Among these, DNA
In an elegant example of conservation of resources, metabolic pathways methylation is a major epigenetic mechanism that is central to the
involving folate are compartmentalized within cells as multienzyme regulation of several cellular functions such as gene transcription,
complexes that shuttle one-carbon units along set paths toward key chromatin structure, imprinting, development, and genomic instabil-
reactions leading to pyrimidine and purine biosynthesis. The major ity. DNA is highly methylated in CpG sequences (over 50%); here a
form of folate transported into the cell is 5-methyl-tetrahydrofolate methyl group is targeted to the DNA base cytosine in the context of
(5-methyl-THF; 5-methy-H 4 PteGlu) (see Fig. 39.5); folic acid, which a CpG dinucleotide by DNA methyltransferases. Methylation confers
is formally called pteroylmonoglutamate (PteGlu), requires reduction a condensed structure and transcriptional repression, whereas hypo-
to tetrahydrofolate by dihydrofolate reductase in a two-step reaction methylation does the opposite. Altered patterns of DNA methylation,
(PteGlu to dihydropteroylglutamate [H 2 PteGlu] to tetrahydropteroyl- particularly hypomethylation involving growth-promoting genes, or
glutamate [H 4 PteGlu, or THF]). After cellular uptake, 5-methyl-THF hypermethylation of tumor suppressor genes, are popular contempo-
must first be converted to THF via methionine synthase (in the rary themes in our understanding of the epigenetic changes in DNA
methylation cycle). This is a key reaction because THF is the preferred and the genesis of cancer. In addition, histone hypomethylation
physiologic substrate for folylpolyglutamate synthase, which adds can alter gene expression. After these transmethylation reactions
multiple glutamate moieties to THF (see Fig. 39.4). Only then can the that use SAM, the immediate product of these reactions is
polyglutamylated form of THF participate in one-carbon metabolism S-adenosylhomocysteine (SAH), which is converted to homocysteine
where it can be converted to either 10-formyl-THF—used in de novo by SAH hydrolase. The SAM to SAH ratio regulates the balance of
biosynthesis of purines, or to 5,10-methylene-THF—used for synthesis such cellular methylation reactions.
of thymidylate. Moreover, 5,10-methylene-THF and 10-formyl-THF Dietary folate deficiency can lead to hypomethylation in experi-
can be interconverted by intermediates (see Fig. 39.5). mental animals, whereas folate consumption supports normal pat-
Folate metabolism and folate-dependent enzymes are compart- terns of methylation. However, analysis in humans has not yielded
mentalized: approximately 40% are in the mitochondrial matrix, consistent results, nor has the correlation of folate status and DNA
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50% in the cytoplasm, and 10% in the nucleus. The methylation methylation been rigorously studied clinically.
reaction occurs in the cytosol whereas thymidylate synthesis occurs in
the nucleus. The mitochondrial compartment not shown in Fig. 39.5
contains its complement of folate cofactors, and homologues of the Consequences of Perturbed One-Carbon Metabolism
major cytosolic enzymes. For example, cytoplasmic 5-methyl-THF
and 5-formyl-THF can enter mitochondria by a mitochondria- Methyl-Folate Trapping
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specific reduced-folate carrier, whereas SAM, which is also required
for mitochondrial methylation reactions, enters mitochondria by a Because of the critical role of methylcobalamin for methionine syn-
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specific transporter. Other one-carbon donors like serine, glycine, thase, a deficiency of cobalamin inactivates methionine synthase and
dimethylglycine, and sarcosine also enter mitochondria and ulti- results in the accumulation of the substrate 5-methyl-THF; this
mately generate formate that crosses back into the cytoplasm. In so-called methyl-folate trap results because the upstream enzyme
the cytoplasm, C1-THF synthase, a trifunctional enzyme, uses this reaction involving methylenetetrahydrofolate reductase (which con-
mitochondria-derived formate with THF to form 10-formyl-THF, verts 5,10-methylene THF to 5-methyl-THF in preparation for the
which is required for the de novo synthesis of purines (see Fig. 39.5); methionine synthase reaction) is irreversible. Because 5-methyl-THF
this enzyme can also catalyze the interconversion of THF, 10-formyl- accumulates and cannot be converted to THF, it leaks out of the cell.
THF, 5,10-methenyl-THF, and 5,10-methylene-THF. In this way, (This explains why patients with cobalamin deficiency can have
the continued delivery of mitochondrial formate helps perpetuate normal to high serum folate values). The ensuing intracellular THF
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cytoplasmic one-carbon metabolism. Another major entry point deficiency compromises one-carbon metabolism and initiates the
of one-carbon units into cytoplasmic folate metabolism is through pathophysiologic cascade leading to perturbed DNA synthesis and
the formation of 5,10-methylene-THF from serine (which is derived megaloblastosis.
from glycolytic intermediates); here the enzyme serine hydroxymeth-
yltransferase catalyzes the addition of carbon 3 from serine to THF
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to give rise to the key intermediate coenzyme 5,10-methylene-THF. Hyperhomocysteinemia
After 5,10-methylene-THF is converted to 5-methyl-THF by the
enzyme methylenetetrahydrofolate reductase, it can be used in Similarly, when methionine synthase is inhibited during either
the methylation cycle that involves methylation of homocysteine cobalamin or folate deficiency, there is a buildup of the thiol amino
via methionine synthase to form methionine and tetrahydrofolate. acid, homocysteine, which can also leak out of the cell and have
After 5,10-methylene-THF is converted (via intermediates) to multiple deleterious effects on the body through a variety of molecular
10-formyl-THF, it can be used for purine nucleotide biosynthesis and biochemical pathways. Indeed, measurement of serum homocys-
involving de novo synthesis of purine nucleotides for DNA and RNA. teine is a sensitive measure of clinical folate and cobalamin deficiency
5,10-Methylene-THF can also be used in the thymidylate cycle via in nutritional anemia. The clinical significance of hyperhomocyste-
the enzyme thymidylate synthase, which generates thymidylate (by inemia is discussed later.
converting deoxyuridine monophosphate [dUMP] to deoxythymi-
dine monophosphate [dTMP]) for DNA synthesis (see Fig. 39.5).
Parenthetically, this nuclear one-carbon metabolism compartment is Thymidylate Deficiency and Perturbed
activated during S phase of the cell cycle, following a posttranslational DNA Synthesis
modification of folate-dependent enzymes like thymidylate synthase,
serine-hydroxymethyltransferase, and dihydrofolate reductase by With either cobalamin or folate deficiency, there will be a net
specialized small ubiquitin-like modifier (or SUMO) proteins; this decrease in 5,10-methylene-THF that interrupts the thymidylate
modification allows the entry and enrichment of these sumoylated synthase–mediated conversion of dUMP to dTMP. (Although salvage
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enzymes into the nucleus to provide a buffer against the stress of pathways for purine synthesis can compensate for reduced generation
imminent folate deficiency. 89 of purines through one-carbon metabolism, salvage pathways cannot