Calcium-dependent human serum homocysteine thiolactone hydrolase. A protective mechanism against protein N-homocysteinylation.
Homocysteine thiolactone is formed in all cell types studied thus far as a result of editing reactions of some aminoacyl-tRNA synthetases. Because inadvertent reactions of thiolactone with proteins are potentially harmful, the ability to detoxify homocysteine thiolactone is essential for biological integrity. This work shows that a single specific enzyme, present in mammalian but not in avian sera, hydrolyzes thiolactone to homocysteine. Human serum thiolactonase, a 45-kDa protein component of high density lipoprotein, requires calcium for activity and stability and is inhibited by isoleucine and penicillamine. Substrate specificity studies suggest that homocysteine thiolactone is a likely natural substrate of this enzyme. However, thiolactonase also hydrolyzes non-natural substrates, such as phenyl acetate, p-nitrophenyl acetate, and the organophospate paraoxon. N-terminal amino acid sequence of pure thiolactonase is identical with that of human paraoxonase. These and other data indicate that paraoxonase, an organophosphate-detoxifying enzyme whose natural substrate and function remained unknown up to now, is in fact homocysteine thiolactonase. By detoxifying homocysteine thiolactone, the thiolactonase/paraoxonase would protect proteins against homocysteinylation, a potential contributing factor to atherosclerosis.
Jakubowski
H
H
Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, New Jersey 07103, USA. jakubows@umdnj.edu
D000596:Amino Acids / Q000494:pharmacology; D001161:Arteriosclerosis / Q000378:metabolism; D043303:Aryldialkylphosphatase; D002118:Calcium / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004950:Esterases / Q000097:blood* / Q000737:chemistry / Q000302:isolation & purification; D006710:Homocysteine / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000378:metabolism; D010396:Penicillamine / Q000494:pharmacology; D017421:Sequence Analysis; D013379:Substrate Specificity
D000596:Amino Acids; D004791:Enzyme Inhibitors; D008075:Lipoproteins, HDL; D006710:Homocysteine; C007957:homocysteine thiolactone; D004950:Esterases; D043303:Aryldialkylphosphatase; D010396:Penicillamine; D002118:Calcium