Irreversible enzyme inhibitors. CLIX. Effect of substitution on transport and isozyme specificity of p-(4,6-diamino-1,2-dihydro-2,2-dimethyl-s-triazin-1-yl)-o-chlorophenoxyacetamido-benzenesulfonyl fluoride, an active-site-directed irreversible inhibitor of dihydrofolic reductase