Substance P inactivation by aqueous humor

Degradation of substance P was studied in dog and rabbit aqueous humor. Substance P inactivation was followed by the bioassay using the isolated guinea pig ileum. Both rabbit and dog aqueous humor inactivated substance P. Rabbit aqueous humor inactivated the peptide faster than dog aqueous humor. Inactivation of substance P by rabbit aqueous humor was inhibited by diisopropylfluorophosphate while other enzyme inhibitors tested (captopril, phosphoramidon, mersalyl acid and p-chloromercuriphenyl-sulphonate) were practically ineffective or had a partial inhibitory effect. Our results suggest that serine proteases, rather than other peptidases, play a major role in the inactivation of substance P in aqueous humor.