Carbamoyl Phosphate Synthetase, Ornithine Transcarbamylase, and Aspartate Transcarbamylase Activities in the Pea Ovary

Abstract

Carbamoyl phosphate synthetase (CPS), ornithine transcarbamylase (OTC), and aspartate transcarbamylase (ATC) were assayed in extracts from unpollinated ovaries of Pisum sativum L. CPS and OTC activities were, per milligram protein, the highest reported in a plant tissue, representing an estimated 0.1% of the protein in the ovary. The OTC/CPS and ATC/CPS ratios were about 100 and 0.5, respectively, indicating that most of the carbamoyl phosphate is used for arginine synthesis. The weight, protein content, and CPS, OTC, and ATC activities per ovary were determined during the senescence of the ovary and also during fruit set induced by treatment with gibberellic acid (GA₃). In the nontreated ovary the weight and the protein first increased and then decreased dramatically, but the decrease in protein took place much earlier. In the GA₃-treated ovaries the increase in weight was considerably greater than the increase in the protein. Whether or not the ovaries were treated with GA₃, CPS, OTC, and ATC activities closely followed the changes in protein, and thus their ratios and specific activities remained essentially constant. It appears that treatment with GA₃ increases the amount of protein and enzymic activities by preventing a large increase in the rate of protein degradation. In addition, the effects of acetylglutamate, ornithine, and UMP on CPS activity were studied. The pea enzyme exhibits regulatory properties intermediate between those of Escherichia coli and the ureotelic liver enzymes.