Homocysteine-Thiolactone - Homocysteine in Protein Structure/Function and Human Disease - page 44

3.5 Turnover

Early experiments show that Hcy-thiolactone infused into rabbits is cleared from
the blood within

15 min [150]. In wild-type mice, intraperitoneally injected Hcy-
thiolactone is quickly hydrolyzed to Hcy, and only a small fraction (6.3 % of the
injected dose) shows up in the plasma [141]. The mouse plasma Hcy-thiolactone is
cleared with a half-life of 5 min [140, 141]. Elevated plasma Hcy-thiolactone in
mice fed with a high-methionine diet is normalized to a basal level after the animals
are shifted to a standard chow diet [113]. Hcy-thiolactone is cleared from the tissues
and blood by specific enzymes and by urinary excretion.

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3.5.1 Enzymatic Turnover

Two enzymes are known to hydrolyze Hcy-thiolactone in humans and animals.
Extracellular hydrolysis is catalyzed by paraoxonase 1 (Pon1) [81, 152, 250], while
intracellular hydrolysis is catalyzed by bleomycin hydrolase (Blmh) [85, 141, 251].
Recent data show that Pon1 and Blmh contribute to Hcy-thiolactone hydrolysis in
the mouse brain [140, 141]. Enzymatic hydrolysis of Hcy-thiolactone to Hcy
facilitates its removal by the classical remethylation and transsulfuration pathways.

3.5.1.1 Paraoxonase 1

Pon1, named for its ability to hydrolyze the organophosphate paraoxon, is a 45-kD
calcium-dependent enzyme synthesized exclusively in the liver and carried on high-
density lipoprotein (HDL) in the blood, although recent studies suggest a wider
expression pattern, including the brain [252]. Pon1 protects against high-fat diet-
induced atherosclerosis in mice [253] and humans [254, 255]. Pon1-deficient mice
are more susceptible to high-fat diet-induced atherosclerosis than wild-type
littermates but do not develop atherosclerosis on a control chow diet [253]. Pon1
is also involved in and affected by Hcy metabolism and thus provides a link
between metabolism of lipids and Hcy [256, 257].
In humans, PON1 is also implicated in Alzheimer's disease. For example, low
serum PON1 activity is a risk factor for dementia [258] and Alzheimer's disease
[259, 260]. Hcy, a risk factor for Alzheimer's disease [56, 57], is a negative
determinant of PON1 activity [261, 262].
In vitro, human HDL and the purified PON1 protein have the ability to hydro-
lyze Hcy-thiolactone [81] and to protect against protein N-homocysteinylation in
serum [152] and cultured human endothelial cells [74]. The in vivo levels of N-
Hcy-protein in human serum are inversely correlated with Hcy-thiolactonase
activity of PON1 [250]. This relationship is recapitulated in in vitro N-homocystei-
nylation experiments with [ 35 S]-Hcy-thiolactone. In Chinese patients with type