Two novel carboxyl-containing arginase substrates, 4-guanidino-3-nitrobenzoic acid and 4- guanidino-2-nitrophenylacetic acid, have been synthesized and found to give enhanced catalysis and dramatically lower Km values relative to 1-nitro-3-guanidinobenzene, a substrate designed for use in a chromophoric arginase assay. To more efficiently mimic the natural substrate, a series of sulfur analogs of l-arginine were synthesized and kinetically ...