Abstract
A constitutively expressed 7 alpha-hydroxysteroid dehydrogenase (7 alpha-HSDH) has been purified over 1200-fold, to apparent homogeneity, from an intestinal anaerobic bacterium. The purified protein had a subunit molecular mass of 32 kDa as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Sepharose CL-6B gel filtration gave a native molecular mass estimate of 124 kDa, suggesting that this enzyme existed as a tetramer of identical subunits. Sulfhydryl reactive compounds were potent inhibitors of 7 alpha-HSDH activity, however, metal ion chelators had no effect upon catalytic activity. The purified enzyme was highly NADP-dependent. Bile acid substrate utilization studies revealed that the enzyme was specific for the oxidation of an unhindered 7 alpha-hydroxyl group. A wide variety of bile acids and analogs were used as substrates including glycine and taurine conjugates, and methyl esters, amines, and bile alcohols. The purified 7 alpha-HSDH obeyed Michaelis-Menten kinetics. Hanes plots of substrate saturation kinetics revealed that most bile acid substrates had Km values ranging from 4 to 20 microM, while Vmax was 601 and 674 mumol/min/mg in the direction of bile acid oxidation and reduction, respectively. Primary kinetic plots and product inhibition patterns were consistent with an ordered sequential mechanism, with NADP(H) binding first. The N-terminal amino acid sequence analysis of the purified enzyme revealed a striking homology to several short, non-zinc alcohol/polyol dehydrogenases and a putative, cholate-inducible, hydroxysteroid dehydrogenase from the same organism. The high specific activity together with the stability, substrate range, and ease of purification, make this enzyme an excellent candidate for use in quantitating primary bile acids both in laboratory and clinical samples. Spectrofluorometry allowed for the quantitation of as little as 10 nM of both free and conjugated primary bile acids.
Highlights
Fold, to apparent homogeneity, from an intestinal anaerobic bacterium
Bile acid substrate utilization studies revealed that the enzyme was specific for the oxidation of an unhindered 7a-hydroxyl group
Hanes plots of substrate saturation kinetics revealed that most bile acid substrates had K, values ranging from 4 to 20 pM, while V, was 601 and 674 pmol/
Summary
Virginia 23298-0678 de Prada, and Phillip B. The high specific activity together with the stability, substrate range, and ease of purification, make this enzyme an excellent candidate for use in quantitating primary bile acids both in laboratory and clinical samples. The enzymes elicited by these organisms are similar in that all have alkaline pH optima for bile acid oxidation and exhibit lower K, values for dihydroxy than for trihydroxy bile acids. They differ with respect to native molecular mass (ranging from 127 to 320 kDa, as judged by gel filtration), and cofactor requirements (either NADP or NAD) [24].
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