The protein A24 content of Ehrlich ascites tumor cells increased several-fold following treatment of cell cultures with nitrosoureas, but did not increase when other alkylating agents not containing carbamoyl moieties were tested. The same nitrosoureas and, in addition, 2-chloroethyl isocyanate inhibited an A24 lyase-containing cytoplasmic extract in cleaving protein A24 into histone H2A and ubiquitin. It appears that carbamoylation of A24 lyase by nitrosoureas inhibits the enzyme and is responsible for the measured increases in cellular protein A24 content due to reduced turnover of this protein.
