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chapter 17
Protein and Amino Acid Metabolism
Folate (F)
-2NADPH+2H+
Dihydrofolate reductase (DR)
2 NADP+
FIGURE 17-18
Homocysteine metabolism.
mutation of C to T at nucleotide position 677 which
causes an alanine-to-valine amino acid substitution in the
protein. The mechanism by which homocysteine medi-
ates vascular pathology remains to be understood. The
targets for homocysteine damage are connective tissue,
endothelial cells, smooth muscle cells, coagulation fac-
tors, nitric oxide metabolism, plasma lipids and their
oxidized forms (Chapter 20). Vitamin supplementation
with B
1 2
, folate, and Bg has reduced total plasma homo-
cysteine levels. Vitamin supplementation may decrease
the morbidity and mortality from atherosclerotic vas-
cular disease due to hyperhomocysteinemia. However,
further studies are required to assess the utility of vitamin
supplementation.
Phenylalanine and Tyrosine
Phenylalanine is an essential amino acid. Tyrosine is syn-
thesized by hydroxylation of phenylalanine and there-
fore is not essential. However, if the hydroxylase sys-
tem is deficient or absent, the tyrosine requirement must
be met from the diet. These amino acids are involved in
synthesis of a variety of important compounds, includ-
ing thyroxine, melanin, norepinephrine, and epinephrine
