Biomedical subjects
Publications and source records attributed to C E Kotts.
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Exposure of guinea pig liver microsomes to phospholipase A2 resulted in the nearly complete loss of 17 beta-hydroxysteroid oxidoreductase (17 beta-HSD) activity, the time course of which correlated with phospholipid hydrolysis and lysolecithin formation. Lysolecithin and unsaturated fatty acids added to microsomes also inactivated 17 beta-HSD indicating that they may contribute to the inactivation by phospholipase A2. If exposure to lysolecithin and fatty acids was minimized by including serum albumin in the reaction mixture, phospholipids were rapidly hydrolyzed; but in this case the extent of 17 beta-HSD inactivation was less and the rate of loss was significantly slower. The data suggest that phospholipid hydrolysis per se results in a destabilization of 17 beta-HSD resulting in the subsequent activity loss. The inactivation of 17 beta-HSD by lysolecithin and fatty acids has not been reported previously and is suggestive of a possible control mechanism in vivo.
When protein is assayed in human placental microsomes by the fluorescamine procedure, 25--30% of the observed fluorescence results from labeling of phospholipids.
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When microsomes were prepared in 2-mercaptoethanol Vmax for 17beta-hydroxysteroid oxidoreductase (17beta-HSD) was greater, the Km for NAD+ was greater and the Km for testosterone lower than in its absence. During storage at 4 degrees Vmax increased in the presence of 2-mercaptoethanol and decreased in its absence; Km values for testosterone and NAD+ increased during storage in both cases. The presence or absence of 2-mercaptoethanol did not affect the extent or time-course of inactivation of 17beta-HSD by trypsin or phospholipase A. Furthermore, no differences were detected in sedimentation properties on sucrose density gradients suggesting that the differences and changes in the kinetic behavior of 17beta-HSD reflect a conformational flexibility at the active site and are not due to extensive changes in the structure of the microsomes. 17beta-HSD exposed to 2-mercaptoethanol was subject to substrate inhibition by testosterone, a type of inhibition not previously reported for this enzyme.
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