Printed in Great Britain HALF-LIVES OF ENZYME ACTIVITIES IN

L5178Y cells (mouse leukaemic cell lines) were grown in Fischer's medium and were utilized in the logarithmic phase of growth. Cells were incubated at 37 °C in 100 fig per ml of cycloheximide. This level of cycloheximide inhibited all cytoribosomal protein synthesis after 30 min incubation. After 70 h incubation with 100 fig per ml of cycloheximide the cell number was, in arbitrary units, 1 07 ±0001 (control: i'63±o-O3), the protein content 161 ±4 pg per cell (control: 181 ± 1) and the percentage viable cells 17 ±2 (control: 9811). Cells were removed at intervals from the incubation in 100 fig per ml of cycloheximide, extracted with 0 1 % Triton X-100, and assayed for various enzyme activities. From these data half-lives, stability times, and decay rates were calculated for each enzyme activity. Lysosomal enzyme activities, in particular the iV-acetyl hexosaminidases, were found to have long half-lives and stability and slow decay rates. Proteolytic enzyme activities were charac-terized by relatively long half-lives and stability times. The membrane marker enzyme activities, especially UDPase, were characterized by extremely short half-lives, rapid decay rates, and no stability time. Mitochondrial enzyme activities had relatively short stability time and half-lives and rather rapid decay rates. The decay occurring at late time intervals was thought to be a consequence of the death phase of the incubated cells as opposed to normal cellular decay