18Oxygen Probes of Protein Turnover, Amino Acid Transport, and Protein Synthesis in Bacillus licheniformis

Experiments were performed in which protein turnover was measured by incorporation of 18O from H218O into protein. Since proteolysis causes the incorporation of 18O into the carboxyl group of amino acids and the incorporation of this amino acid leads to protein with the peptide carboxyl oxygen labeled, turnover can be determined quantitatively. The rate of protein turnover during the first 5 hours after growth was found to be about 22% per hour. Spores contain an amount of 18O that allows the suggestion that the average protein turns over two or more times during sporulation. To examine the possibility of oxygen exchange during protein synthesis, Bacillus licheniformis cells were grown through eight generations, a 200-fold increase in mass, on a medium of glucose plus a mixture of amino acids that were labeled with 18O in the carboxyl groups. The protein of the cells contained oxygen, after appropriate corrections, at an atom per cent excess similar to that of the added amino acids. It is concluded that neither amino acid transport nor protein synthesis involve mechanisms that lead to significant loss or exchange of amino acid carboxyl oxygens.