Enamine formation of various amino acids with ethyl acetoacetate in aqueous solution in relation to colonic absorption of cefmetazole.

Abstract

Amino acids such as DL-Ala, Gly and L-Leu form the enamine with ethyl acetoacetate even in aqueous solution, as determined by ultraviolet (UV)-spectrophotometry. Although the observed formation constant, Kobs, of the enamine from each amino acid showed a dependency on pH, the values of the intrinsic enamine formation constant, K, were rather similar for all the amino acids and showed no dependence on pH. A solution containing a mixture of ethyl acetoacetate and an amino acid increased the colonic absorption of cefmetazole significantly. The extent of the absorption enhancement depended on the pKa of the amino group of the amino acid contained in the test solution.