Digestion of peptidic residues in humic substances by an alkali-stable and humic-acid-tolerant proteolytic activity in the gut of soil-feeding termites

Previous feeding experiments have shown that soil-feeding termites (Termitidae: Termitinae) preferentially mineralize the peptidic component of synthetic humic acids, but nothing was known about the mechanism involved in digestion. Here, we studied the hydrolysis of humus-stabilized peptides in gut extracts of Cubitermes orthognathus by measuring the release of radiolabel from ¹⁴C-peptide-labeled synthetic humic acids. Gut extracts exhibited proteolytic activity over a wide pH range (from 4 to 12) with a maximum at about pH 8. The highest activity was located in the gut section containing the midgut and the extremely alkaline (up to pH 12) mixed segment. Chemical hydrolysis at in situ pH (up to pH 12) was negligible. Proteolytic activity in the hindgut fluid was generally relatively low, but alkaline proteases dominated in the anterior hindgut. When compared to other alkaline proteases, the proteolytic activity of gut extracts had a higher alkali-stability and tolerance to humic acids than subtilisin and an alkaline protease of Streptomyces griseus. Gut extracts also hydrolyzed the peptidic component of synthetic humic acids more efficiently than the commercial enzymes. Together with previous results, this study strongly supports the hypothesis that soil-feeding termites mobilize and digest the peptidic component of organic nitrogen in soil humic substances by a combination of proteolytic activities and extreme alkalinity in their intestinal tract.