Sorbitol dehydrogenase inhibitors have been found to prevent, or alleviate, various secondary
complications of diabetes mellitus. In the present study, the effects of
nucleosides and
nucleotides on the rate of
sorbitol oxidation catalyzed by the sheep liver
enzyme were studied by steady-state kinetics at pH 7.4. Various such compounds, including
ATP and the 2'-deoxy-analogues of
ATP,
ADP and
AMP, reversibly inhibit
enzyme activity by formation of
enzyme-
coenzyme-inhibitor ternary complexes. In each case, no deviations from linearity were seen in the double-reciprocal plots using
sorbitol or
NAD(+) as the varied substrate and there was a linear relationship between inhibitor concentration and the observed inhibitory effects.
Sorbitol was docked into a model of the sheep SDH-
NAD(+) complex based upon the structure of the human SDH-
NAD(+)
holoenzyme. The resulting structure of the ternary complex of sheep SDH,
NAD(+) and
sorbitol (PMDB ID code PM 0078068) shows that the reactive C-2
hydroxyl group of
sorbitol is oriented toward the 4'-position of the
nicotinamide moiety of the
coenzyme, and that the adjacent primary
hydroxyl group of
sorbitol interacts with the catalytic
zinc. The results indicate that the
ribose moiety of the inhibitor structures is an important determinant for the observed effects. Specifically, the 2'-position of the
ribose ring exerts an effect with respect to inhibitor potency.