Hydrolytic deactivation of kyotorphin by the rodent brain homogenates and sera

A simple analytical assay method for kyotorphin was developed by the use of amino acid analyzer and the deactivation of kyotorphin in biological media was studied with this method. Kyotorphin was hydrolyzed rapidly by enzymes in serum and brain of rat and mouse, but L-tyrosyl-D-arginine was not. The hydrolysis activity was much higher in brain than in serum. The apparent Michaelis' constants, Km, were of the order of 10(-5) for brain and 10(-4) M for serum in both species. The hydrolysis of kyotorphin was strongly inhibited by bestatin (Ki = 0.1 microM). Thus the hydrolysis might be attributed to aminopeptidase(s). A dose of 50 micrograms of bestatin administered intracisternally potentiated the analgesic activity of kyotorphin by 4.8 times or so. Thus kyotorphin seems to be deactivated by bestatin-sensitive aminopeptidase(s) in brain.