Parenteral administration of sericystatin A, an intracellular leukocyte inhibitor of the proinsulin-splitting enzyme cathepsin B, prolongs the mean survival time of mice with insulin-dependent myeloid leukemia. Sericystatin A reduces the supranormal blood levels of substances detectable by insulin-specific radioimmunoassay in these animals; this effect is abolished by concomitant insulin administration. It is concluded that sericystatin A reduces tumor proliferation indirectly, i.e. by reducing circulating insulin levels.
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