Metabolism and neurotoxicity of homocysteine thiolactone in mice: protective role of bleomycin hydrolase

Abstract

Genetic or nutritional disorders in homocysteine (Hcy) metabolism elevate Hcy-thiolactone and cause heart and brain diseases. Hcy-thiolactone has been implicated in these diseases because it has the ability to modify protein lysine residues and generate toxic N-Hcy-proteins with auto-immunogenic, pro-thrombotic, and amyloidogenic properties. Bleomycin hydrolase (Blmh) has the ability to hydrolyze L-Hcy-thiolactone (but not D-Hcy-thiolactone) to Hcy in vitro, but whether this reflects a physiological function has been unknown. Here, we show that Blmh (-/-) mice excreted in urine 1.8-fold more Hcy-thiolactone than wild-type Blmh (+/+) animals (P = 0.02). Hcy-thiolactone was elevated 2.3-fold in brains (P = 0.004) and 2.0-fold in kidneys (P = 0.047) of Blmh (-/-) mice relative to Blmh (+/+) animals. Plasma N-Hcy-protein was elevated in Blmh (-/-) mice fed a normal (2.3-fold, P < 0.001) or hyperhomocysteinemic diet (1.5-fold, P < 0.001), compared with Blmh (+/+) animals. More intraperitoneally injected L-Hcy-thiolactone was recovered in plasma in Blmh (-/-) mice than in wild-type Blmh (+/+) animals (83.1 vs. 39.3 μM, P < 0.0001). In Blmh (+/+) mice injected intraperitoneally with D-Hcy-thiolactone, D,L-Hcy-thiolactone, or L-Hcy-thiolactone, 88, 47, or 6.3%, respectively, of the injected dose was recovered in plasma. The incidence of seizures induced by L-Hcy-thiolactone injections (3,700 nmol/g body weight) was higher in Blmh (-/-) than in Blmh (+/+) mice (93.8 vs. 29.5%, P < 0.001). Using the Blmh null mice, we provide the first direct evidence that a specific Hcy metabolite, Hcy-thiolactone, rather than Hcy itself, is neurotoxic in vivo. Taken together, our findings indicate that Blmh protects mice against L-Hcy-thiolactone toxicity by metabolizing it to Hcy and suggest a mechanism by which Blmh might protect against neurodegeneration associated with hyperhomocysteinemia and Alzheimer's disease.

Homocysteine and asymmetric dimethylarginine concentrations in the plasma of Alzheimer’s disease patients with varying degrees of dementia

“…In present study both aging and the degenerative diseases have been shown to be accompanied by lowered levels of Met and Met to Hcy ratio, most probably due to disturbed process of Hcy remethylation. At the same time, according to a recent theory, the lowered ratio of Met to Hcy may be linked to induction of arteriosclerotic lesions by Hcy with involvement of thiolactone [19]. One of the products of Met demethylation is known to involve S-adenosylmethionine (SAM).…”

Functionally Defective High-Density Lipoprotein and Paraoxonase: A Couple for Endothelial Dysfunction in Atherosclerosis

“…Named for its ability to hydrolyze organophosphates like paraoxon found in insecticides, PON1 is also able to hydrolyze N-acyl-homoserine, a lactone used by pathogenic bacteria, and lipid peroxides, thereby inhibiting the formation of foam cells known to contribute to atherosclerosis [16, 20–22]. …”

Homocysteine in Protein Structure/Function and Human Disease

“…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]. Furthermore, Hcy-thiolactone is known to be more toxic than Hcy to cultured human endothelial cells [61] and to mice [140,141]. Furthermore, Hcy-thiolactone is known to be more toxic than Hcy to cultured human endothelial cells [61] and to mice [140,141].…”