Protease activities in carp retina

Proteases of carp retina were examined by electrophoresis and fluorogenic assays. A 70 kD serine protease with an alkaline pH optimum was detected in gelatin-containing polyacrylamide gels. A similar enzyme was found in carp brain and muscle, but not in lens. Using aminomethylcoumarin (MCA) substrates, activities that hydrolysed Z-Phe-Arg-MCA, Boc-Ala-Gly-Pro-Arg-MCA and various aminoacyl-MCAs were detected. The Z-Phe-Arg-MCA hydrolase was an acidic cysteine protease, whereas the Boc-Ala-Gly-Pro-Arg-MCA hydrolase was an alkaline cysteine protease. All aminoacyl hydrolase activities tested were inhibited by bestatin and o-phenanthroline, but not by inhibitors of serine, cysteine and aspartic proteases, suggesting they are metalloaminopeptidases. Of the substrates tested, Tyr-MCA was the most readily hydrolysed aminoacyl substrate. Preliminary evidence was obtained suggesting that levels of these activities do not differ between light- and dark-adapted retinae.

The proteases have a potential involvement in retinal functioning and show similarities to other proteases known to act in the central nervous system. In particular, the Tyr-MCA hydrolase may be related to an enzyme known to remove the N-terminal tyrosine residue from enkephalin.