An Overview of Homocysteine Metabolism - Homocysteine in Protein Structure/Function and Human Disease - page 7

Hcy), inadequate supply of CH 3 -THF, or impairment of remethylation or transsul-
furation reactions by genetic alterations of enzymes, such as CBS, MS, or MTHFR.
Hcy-thiolactone is neutral at physiological pH (pK a ¼ 6.67, [84]) and thus

accumulates mostly in the extracellular fluids.
Hcy-thiolactone is metabolized by two pathways: (1) a hydrolytic pathway that
affords Hcy—this pathway is catalyzed by intracellular and extracellular Hcy-
thiolactonases, otherwise known as bleomycin hydrolase (Blmh) [85] and
paraoxonase 1 (Pon1) [81], respectively; and (2) a synthetic pathway in which
Hcy-thiolactone reacts chemically with protein lysine residues, forming N-Hcy-
protein [78, 79, 81] (Fig. 1.1 ).
Proteolytic degradation of N-Hcy-protein affords Nε -Hcy-Lys (Fig. 1.1 ), first

discovered in a biological system when N-Hcy-hemoglobin was incubated with
liver extracts [72]. The isopeptide Nε

-Hcy-Lys is present in human and mouse
plasma. Its levels increase in renal disease and CBS deficiency in humans. Genetic
deficiencies in folate and Hcy metabolism and a hyperhomocysteinemic high-Met
diet cause elevation of Nε -Hcy-Lys are

positively correlated with the nitric oxide synthase inhibitor asymmetric
dimethylarginine [86], which is consistent with their common origin as products
of protein turnover [72].

-Hcy-Lys in mice. The plasma levels of Nε

2.1 Homocysteine Metabolite Levels

In addition to the free thiol (reduced) form, i.e., free Hcy-SH, several other Hcy
species are present in human and animal plasma (Table 2.1 ) [87]. These include the
disulfide homocystine Hcy-S-S-Hcy identified in the urine of patients with
homocystinuria [88]; a mixed disulfide of Hcy and Cys, Hcy-S-S-Cys, first
described in patients with cystinuria [89] and then in patients with homocystinuria
[90]; and a mixed disulfide of Hcy with plasma protein, S-Hcy-protein [91, 92], as
well as species with substituted carboxyl groups, such as Hcy-thiolactone [64, 76,
93-95], N-Hcy-protein [76, 79, 96], and Nε

-Hcy-Lys [72]. Other known Hcy
metabolites include AdoHcy, cystathionine [97, 98], homocysteine sulfinic acid,
and homocysteic acid (Table 2.1 ) [20, 99].
S-Nitroso-Hcy is a potential metabolite that has been first detected in bovine
aortic endothelial cells incubated with exogenous Hcy [100]. Treatment of these
cells with the nitric oxide synthase inhibitor N-nitro- L -Arg prevents nitric oxide
generation and the formation of S-nitroso-Hcy. Increasing nitric oxide generation
by overexpression of the inducible nitric oxide synthase in endothelial cells results
in a significant increase in the formation of S-nitroso-Hcy [101]. That human
endothelial cells produce S-nitroso-Hcy has also been inferred from its ability to
mediate translational incorporation of Hcy into protein [75] that is observed in these
cells [68, 76, 83]. However, the presence of S-nitroso-Hcy in humans or animals
remains to be documented.