An Overview of Homocysteine Metabolism - Homocysteine in Protein Structure/Function and Human Disease - page 12

2.2.1 Free Reduced Homocysteine

There is some evidence suggesting that the free thiol Hcy (reduced form) is more
harmful than the disulfide Hcy species (oxidized forms). For example, in two studies
that examined the toxicity of individual Hcy species, the free thiol Hcy was associated
with endothelial dysfunction in humans, whereas much more abundant disulfide Hcy
species were not [136, 137]. The toxicity of free thiol Hcy most likely results from its
metabolic conversion to Hcy-thiolactone, which occurs in cultured human and rodent
cells [68, 73, 77, 138, 139] and in human [64, 76, 94, 95] and mouse bodies [93, 113].
Furthermore, Hcy-thiolactone is known to be more toxic than Hcy to cultured human
endothelial cells [61] and to mice [140, 141]. That the metabolic conversion of Hcy to
Hcy-thiolactone is responsible for Hcy toxicity is further supported by studies with rat
embryos, which show that L -Hcy-thiolactone, D - Hcy-thiolactone, and L -Hcy are toxic,
whereas metabolically inactive D -Hcy (which is not recognized by methionyl-tRNA
synthetase and thus cannot be metabolized to Hcy-thiolactone) is not [142]. Plasma
levels of the free thiol Hcy are maintained low by its oxidation to disulfides, mostly
with the major plasma proteins albumin [81, 103] and γ-globulins [79, 106].

2.2.2 S -Homocysteinyl-Protein

There is no evidence that any of the disulfide species of Hcy that occur in the
plasma is harmful to the human body [143]. In fact, only free reduced Hcy,
comprising 1-2 % of tHcy, but not any of the more abundant disulfide forms of
Hcy, including S-Hcy-protein, comprising 80 % of tHcy, is associated with dimin-
ished vascular function as measured by flow-mediated dilatation [136, 137]. How-
ever, that some S-Hcy-proteins may be harmful is suggested by findings showing
that treatments of human endothelial cells with in vitro-prepared S-Hcy-ApoB-100
reduce cell proliferation and viability [144].
In a mouse model of dietary hyperhomocysteinemia, Hcy appears to inhibit
vascular fibrinolysis in vivo by forming a disulfide bond with annexin A2 thiol
[145]. Annexin A2, an endothelial cell surface co-receptor for plasminogen and
tissue plasminogen activator, accelerates the catalytic activation of plasmin, the
major fibrinolytic agent in mammals. Thus, derivatizing annexin A2 by Hcy
contributes to prothrombotic effects of hyperhomocysteinemia [145].
Whether the formation of intracellular S-Hcy-protein in humans could be
harmful has been addressed in an ex vivo study, which suggests that Hcy binding
to metallothionein, an intracellular zinc-binding protein, causes elevation of intra-
cellular free zinc levels and oxidative stress in cultured human endothelial cells
treated with exogenous Hcy [146]. Unfortunately, whether the proposed Hcy
binding involves a disulfide bond formation with metallothionein has not been
confirmed with purified protein. Although these effects were ascribed to the forma-
tion of a putative S-Hcy-metallothionein disulfide, the authors' observations that