Potential Influence of Midgut pH and Redox Potential on Protein Utilization in Insect Herbivores

Early insect physiologists recognized the importance of gut physicochemistry, primarily pH, redox potential, and ionic strength, on digestive processes, but studies to date have been primarily restricted to keratin- and wood-feeding insects. Recent investigations show that herbivorous insects have a broad range of gut redox conditions, with pHs ranging from 6.0 to 11.8 and measured redox potentials from -200 to +240 mV. The redox state of the gut is largely dependent on pH, which is well regulated, and the redox activity of ingested material, including plant chemicals, at that pH. Inter- and intraspecific variation in midgut redox conditions appears to be substantial enough to affect digestion via effects on the structure and function of dietary proteins and proteolytic enzymes. The impact of reducing conditions on proteins probably depends on characteristics such as tertiary structure and the number and arrangement of disulfide linkages. In addition to the effects of reducing conditions in dietary proteins, there can be effects on the activities of digestive enzymes, depending on their structure and the nature of their catalytic site. We speculate that phylogenetic and environmental determinants of gut physicochemistry may place constraints on the efficacy of different digestive processes, and may thus influence the evolution of digestive strategies in insects.