Aromatic boronic acids are reversible inhibitors of the recently classified class C beta-lactamases. The boronic acids studied include ortho-, meta- and para-methyl-, -hydroxymethyl- and -formyl-phenylboronic acid. The beta-lactamases were chromosomally-encoded enzymes, one from Pseudomonas aeruginosa, and the other specified by the ampC gene of Escherichia coli. The inhibition may be correlated with our finding that these beta-lactamases are serine enzymes, i.e. their function entails the hydroxy group of a serine residue acting as a nucleophile.

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MATERIALS AND METHODS Crude cholera toxin, NIH-001 containing about 1 bluing dose (1) per ,Ag, was supplied by the (then) Cholera Advisory Committee of the National Institutes of Health. The toxin was always dissolved in 0.2 M phosphate buffer (pH 7) containing 0.2% gelatin. Guiinea pig and rabbit tissues were homogenized with water by hand with a glass homogenizer, and subsequent dilutions of the homogenates were made in water, except in experiments involving pretreatment with enzymes. With sialidase, the diluent was 0.1 M acetate buffer (pH 6), and with protease it was 0.2 M

Endongenous and exogenous sialidases appear to unmask sialidase-stable and sialidase-labile gangliosides in intestinal mucosal homogenates by attacking glycoproteins. Exogenous (but not endogenous) sialidase then converts sialidase-labile gangliosides into the cholera toxin-binding, sialidase-stable ganglioside GM1 (galactosyl-N-acetylgalactosaminyl [sialosyl] lactosyl ceramide). Since Vibrio cholerae produces sialidase, these observations may be relevant to the course of cholera.