Pyridoxal-5'-phosphate derivatives of substance P: preparation, spasmogenic activity, and degradation by human plasma.

Two fluorescent derivatives of substance P (SP) (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2) were prepared by chemical modification of the native peptide by pyridoxal-5'-phosphate (pyridoxal-P). The formation of both pyridoxal-P-derivatives of SP is the result of one modification procedure. The determination of the amino acid composition showed that in one of the derivatives the epsilon-amino group of the Lys residue [epsilon-(P-pxy)-SP] and in the other the epsilon-amino group of the Lys residue and also the N-terminal amino group [alpha, epsilon-di-(P-pxy)-SP] of SP had been substituted by pyridoxal-P. epsilon-(P-pxy)-SP and alpha, epsilon-di-(P-pxy)-SP have spasmogenic activity with ED50 of 1.8 X 10(-9) and 4 X 10(-9) M, respectively, tested on isolated guinea pig ileum. The fluorescence of P-pxy residues permits detection of as little as 1 pmol/ml of epsilon-(P-pxy)-SP and 0.5 pmol/ml of alpha, epsilon-di-(P-pxy)-SP. Both analogues of SP obtained are degraded by human plasma more slowly than the native peptide.