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The study of these systems is the most advanced and has yielded information about reactivity and labeling protocols that is applicable to the study of other steroid hormone binding sites. J. , 1976)] causes the isomerization of A5-3-ketosteroids to the AMsomers. The bacterial enzyme, isolated from testosteroneinduced Pseudomonas testosteroni, is soluble and is readily purified by conventional and affinity Chromatographie techniques. , 1976). Its substrate specificity is such that 19-norandrostenones are most rapidly isomerized; A4-3-ketosteroids are competitive inhibitors.

The isomerase became covalently labeled (determined by Sephadex G-25 gel filtration) when it was incubated with [ 3 H]la; the labeling appeared to be reasonably specific, as the extent of inactivation roughly paralleled the degree of covalent incorporation of the steroid. When the time course of the inactivation-labeling process was monitored electrophoretically, the progressive formation of at least three labeled, electrophoretically distinct species was observed. It is not yet clear whether these represent species labeled to different degrees with [ 3 H]la or present in different states of aggregation.

Its substrate specificity is such that 19-norandrostenones are most rapidly isomerized; A4-3-ketosteroids are competitive inhibitors. This enzyme has been the subject of three affinity labeling studies. Buki et al. (1971) found that 6ß-bromotestosterone acetate (la), a 0-R 0-R la lb X Y Z R Br H H C0CH 3 H Br H COCH3 X Y R 2a Br H H 2 b Br Br C0CH 3 le H 2c H Br H H Br COCH3 potent competitive inhibitor (Κ4 = 57 μΜ), caused irreversible inactivation of the enzyme upon long-term incubation. 2ξ,6^-Dibromoandrost-4-ene-3,17-dione (a mixture of isomers) also caused inactivation, but it was a weaker competitive inhibitor and was not further studied.