A novel chemical modification of delta 5-3-ketosteroid isomerase occurring during its 3-oxo-4-estren-17 beta-yl acetate-dependent photoinactivation.

The chemical change responsible for the 3-oxo-4-estren-17 beta-yl acetate-dependent photoinactivation of delta 5-3-ketosteroid isomerase has been identified by amino acid analysis and amino acid sequencing. Amino acid analysis of the enzyme and its photoinactivated derivative shows that photoinactivation is accompanied by loss of nearly 1 residue of aspartic acid/polypeptide chain and an increase in nearly 1 residue of alanine. Edman degradation of a peptide comprising residues 31 to 48 from native isomerase showed the presence of aspartic acid at residue 38. When the corresponding peptide from photoinactivated enzyme was sequenced, residue 38 was revealed to be alanine.