Stimulation of tyrosine aminotransferase degradation by methylthioinosine

Methylthioinosine (MeSno) is a purine nucleoside analog which is cytotoxic to a number of cultured cell lines including the Reuber H35 hepatoma cells used in the present studies. It has also been observed to cause a rapid profound loss of tyrosine aminotransferase activity in H35 cells well before the onset of any measurable cytotoxicity. The effect is both time and concentration dependent. MeSno does not acutely inhibit synthesis of the enzyme as evidenced by the ability of glucocorticoids or cAMP analogs to induce the enzyme to the same extent in the presence or absence of the drug. The enzyme in extracts of cells treated with the drug is essentially identical with the enzyme from extracts of control cells in terms of thermal stability, immunoprecipitability, and affinities for substrates and cofactor. Addition of MeSno to cell extracts and mixing experiments suggests that the thiopurine does not have any direct effect on enzyme activity. Immunochemical analysis of the rates of synthesis and degradation of the aminotransferase have shown that the enzyme is degraded approximately 3-4 times more rapidly in cells treated with the drug than in control cells. At the same time there is no inhibition of the rate of synthesis of the enzyme.