Dipeptidase activities in rat brain synaptosomes can be distinguished on the basis of inhibition by bestatin and amastatin: identification of a kyotorphin (Tyr-Arg)-degrading enzyme.

The neuropeptide kyotorphin (Tyr-Arg) was degraded by rat brain synaptosomes via a synaptic membrane-bound peptidase which was inhibited by bestatin but not by amastatin. The Km for kyotorphin was 8 x 10(-6) M and the Ki for bestatin was 1 x 10(-7) M. The kyotorphin-degrading enzyme was distinguished from at least one other dipeptide-hydrolyzing activity in synaptosomes which was inhibited by both bestatin and amastatin. Gel permeation chromatography of detergent-extracted synaptosomes resulted in the separation of the dipeptide-hydrolyzing activities. A single kyotorphin-degrading enzyme peak was observed which had a M(r) = 52,000. The activity peak could degrade other dipeptides including Phe-Arg, a synaptic membrane-generated metabolic of bradykinin. The kyotorphin-degrading enzyme appears to be novel and can be distinguished from other known dipeptidases on the basis of substrate specificity, subcellular localization, and inhibition profile.