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A series of leupeptin analogs R-L-leucyl-L-leucyl-L-argininal with variable N-terminal substituents has been synthesized using N alpha-tert-butyl-oxycarbonyl-NG-benzyloxycarbonyl-L-arginine-delta-lactam as the starting material. The modified leupeptins proved to be strong competitive inhibitors of the endoprotease acrosin from mammalian spermatozoa. Inhibition constants were found in the range of 4.7 X 10(-7)M (R = H) to 9.7 X 10(-9)M (R = tert-butyloxycarbonyl). N alpha-tert-butyloxycarbonyl leupeptin represents the strongest acrosin inhibitor synthesized so far. Two of the leupeptin derivatives (R = trifluoroacetyl, R = tert-butyloxycarbonyl) were more effective than the natural leupeptins from microbial sources (Ki = 5.9 X 10(-8)M). The potential use of synthetic leupeptins as antienzymatic contraceptives is discussed.

Synthesis of Leupeptins and Inhibition of Proteinases. I. Inhibition of Acrosin and Trypsin