Irreversible enzyme inhibitors. CXXIX. p-(4,6-Diamino-1,2-dihydro-2,2-dimethyl-s-triazine-l-yl)phenylpropionylsulfanilyl fluoride, an active site-directed irreversible inhibitor of dihydrofolic reductase. 5. Effects of substitution on the benzenesulfonyl fluoride moiety on isozyme specificity