Studies on the structure and properties of sheep liver sorbitol dehydrogenase
Sorbitol dehydrogenase (SDH) from sheep liver was purified by chromatography on the affinity media Blue Sepharose and Matrex Red and also on the ion-exchange medium, carboxymethyl-cellulose. The molecular weight of the native enzyme was about 140,000 (found by gel filtration), and that of the subunit was 35,000-40,000 (estimated by sodium dodecyl sulphate polyacrylamide gel electrophoresis). Mess values suggest a tetrameric arrangement for the quaternary structure. SDH could react with NADP+ or NADPH but at a much lower rate than with NAD+ or NADH. The enzyme reacted, at low rates, with propan-l,2-diol and glycerol (with NAD+ as coenzyme) and with hydroxypropan-2-one and 2,3-dihydroxypropan-2-one (with NADH as coenzyme). Inhibition by EDTA and 1,10-phenanthroline suggested the presence of an essential metal. Enzyme lability in solutions lacking free thiols and inhibition by N-ethylmaleimide and iodoacetamide indicated the presence of essential cysteine(s). Inhibition by diethylpyrocarbonate suggested that histidine was necessary for activity. Carboxymethylation of SDH under non-denaturing conditions revealed one highly reactive cysteine. This residue could be protected from labelling by NADH but not sorbitol. Fully carboxymethylated SDH was subjected to proteolytic digestion by cyanogen bromide and by trypsin. The peptides obtained were purified by gel filtration, ionexchange chromatography, HPLC, paper chromatography and high voltage paper electrophoresis. A 45-residue sequence of amino acids around the reactive cysteine was determined and revealed similarities to the equivalent regions of horse liver and yeast alcohol dehydrogenases suggesting distant evolutionary relationships.