Studies on the structure of rat liver ornithine aminotransferase.

Two distinct types of apoenzyme from native ornithine aminotransferase [EC 2.6.1.13], named forms I and II, have been found to be composed of a single type of subunit by means of Sephadex G-100 column chromatography in the presence of sodium dodecylsulfate, DE 52 cellulose column chromatography in the presence of 8 M urea and urea-polyacrylamide gel disc electrophoresis. The amino-terminal residues of both apoenzymes were blocked, but several amino acids in sequence from the carboxylterminals were identical and the following sequence was found in both: Phe-Ser-Leu-IIe-Ala-(Val)-. In the native enzyme, 4 SH groups out of a total 20 SH groups/tetramer could be titrated without loss of enzyme activity, while in the apo-form I, 4 SH groups out of a total 20 SH groups/tetramer could be titrated with 80% loss of activity. In the apo-form II, in contrast, 6 SH groups out of a total 10 SH groups/dimer could be titrated and titration of the first 2 SH groups caused 100% loss of activity.