Studies on functions of reductones. XX. Effect of reductones on glyoxalase I.

Abstract

SummaryThe effect of some reductones on glyoxalase I prepared from animal and microbial origins has been studied. The enzyme was extracted from ox liver or baker's yeast and partially purified by am monium sulfate fractionation, gel filtration and ion exchange chromato graphy. Aliphatic reductones such as ascorbic acid, ascorbic acid 3 -phosphate and triose reductone showed strong to medium inhibition, while dehydroascorbic acid showed no inhibition. Kinetic analysis indicated that the inhibition mechanism of ascorbic acid was uncom petitive. Varying extents of inhibition were observed among three kinds of diphenols belonging to aromatic reductones. They were in the order of increasing inhibitory power resorcinol, hydroquinone and catechol for the ox liver enzyme, and catechol, resorcinol and hydro quinone for the yeast enzyme. p-Benzoquinone, an oxidized reductone, exhibited marked inhibition on both enzymes. Its action seemed due to reaction with amino and/or sulf hydryl functions of enzyme protein and those of glutathione, one of the substrates.Glyoxalase I [S-lactoyl-glutathione methylglyoxal-lyase (isomerizing), EC 4.4.1.5] is widely distributed in nature and catalyzes the conversion of methyl glyoxal and reduced glutathione to S-lactoyl-glutathione. A true substrate of the enzyme is now presumed to be hemimercaptal formed nonenzymatically from the a-ketoaldehyde and glutathione. The role of the enzyme was considered to be the detoxification of methylglyoxal until 1968, when SZENT-GYORGYI (1) pro posed a mechanism controlling cell proliferation, where he assigned methylglyoxal to a factor retarding cell division, and glyoxalase I to a promoting agent of cell multiplication by transforming the a-ketoaldehyde.A group of compounds, classified as reductones, were examined and shown to have antitumor potency by cleaving DNA strands of tumor cells at appropriate positions (2-4). They include biologically important substances such as ascorbic 1 Studies on Functions of Reductones , XX. 2

Potential transition-state inhibitors of glyoxalase. I

“…Dehydroascorbic acid at 0.7 mM could only be estimated as about 5% inhibitory. These findings are similar to those reported by Iio and co-workers [28] for yeast glyoxalase I, where 0.6 mM ascorbic acid inhibited at 50% and dehydroascorbic acid was not inhibitory at 6 mM. Liver glyoxalase I inhibition values were slightly different, but I 50 concentrations were not reported.…”