Half-lives of enzyme activities in an l5178y mouse leukaemic cell

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 pg per ml of cycloheximide. This level of cycloheximide inhibited all cytoribosomal protein synthesis after 30 min incubation. After 7·0 h incubation with 100 μg per ml of cycloheximide the cell number was, in arbitrary units, 1·07 ±0·001 (control: 1·63 ±0·03), the protein content i6i±4pg per cell (control: 181 ± 1) and the percentage viable cells 17 ±2 (control: 98 ±1). Cells were removed at intervals from the incubation in 100 μg 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 jV-acetyl hexosaminidases, were found to have long half-lives and stability and slow decay rates. Proteolytic enzyme activities were characterized 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.