The mode of action and the structure of a herbicide in complex with its target: binding of activated hydantocidin to the feedback regulation site of adenylosuccinate synthetase.

(+)-Hydantocidin, a recently discovered natural spironucleoside with potent herbicidal activity, is shown to be a proherbicide that, after phosphorylation at the 5' position, inhibits adenylosuccinate synthetase, an enzyme involved in de novo purine synthesis. The mode of binding of hydantocidin 5'-monophosphate to the target enzyme was analyzed by determining the crystal structure of the enzyme-inhibitor complex at 2.6-A resolution. It was found that adenylosuccinate synthetase binds the phosphorylated compound in the same fashion as it does adenosine 5'-monophosphate, the natural feedback regulator of this enzyme. This work provides the first crystal structure of a herbicide-target complex reported to date.

Anticancer drugs as inhibitors of two polymorphic cytochrome P450 enzymes, debrisoquin and mephenytoin hydroxylase, in human liver microsomes.

To identify potential substrates for the debrisoquin and mephenytoin hydroxylation polymorphisms, we performed in vitro inhibition studies with human liver microsomes and the respective prototype substrates in the absence and presence of several anticancer drugs. (+)-Bufuralol 1'-hydroxylation (as the prototype reaction for the debrisoquin polymorphism) was tested at 5 microM substrate concentration and in the presence of cyclophosphamide (0 to 200 microM), teniposide (0 to 100 microM), vinblastine (0 to 220 microM), etoposide (0 to 200 microM), flavone acetic acid (0 to 1000 microM), or ifosphamide (0 to 200 microM). (S)-Mephenytoin 4-hydroxylation was tested at 60 microM substrate concentration and in the presence of the same drugs as above; vincristine was also tested at 0 to 200 microM. Teniposide competitively inhibited the 4-hydroxylation of (S)-mephenytoin, with a Ki of 12 microM (Km of the reaction = 65 microM). Etoposide and flavone acetic acid were weaker inhibitors of this reaction. The only agent to inhibit bufuralol hydroxylation was vinblastine, which did so with a Ki of 90 microM (Km of the enzyme for the substrate = 12 microM). We conclude that teniposide and high concentrations of flavone acetic acid could spuriously alter mephenytoin phenotype determination in cancer patients, and that teniposide deserves further investigation as a possible substrate for the genetically regulated mephenytoin hydroxylase.