The membrane-bound intestinal enzymes of waxwings and thrushes: Adaptive and functional implications of patterns of enzyme activity

Cedar waxwings (Bombycilla cedrorum) feed predominantly on fruits that are rich in simple sugars and low in nitrogen, supplementing this diet with art hropod prey during the summer months as well as flowers and tree sap in spr ingtime. In contrast, thrushes feed extensively on fatty, protein-rich inve rtebrate prey, supplemented with sugary and lipid-rich fruits. Simple sugar s and fats are digested and/or absorbed by distinctly different physiologic al mechanisms, which suggests the possibility of contrasting digestive stra tegies in animals specialized to diets containing one of these two energy s ources. In this study, we quantified enzymatic activity of three membrane-b ound intestinal enzymes of cedar waxwings and five species of thrushes to e xplore this aspect of their digestive physiology. These enzymes catalyze th e final steps in the digestion of carbohydrates (sucrase-isomaltase and mal tase-glucoamylase) and protein (aminopeptidase-N). The two carbohydrases ar e homologous enzymes with overlapping functions; both enzymes catalyze the hydrolysis of maltase and isomaltase. The membrane-bound digestive enzyme s ystems that we described for cedar waxwings and thrushes can be explained b y the particular nutrients contained within their respective natural diets. Consistent with previous work, cedar waxwings displayed intestinal sucrase activity, whereas thrushes did not. Correspondingly, cedar waxwings eat so me foods containing sucrose, whereas thrushes do not. Sucrase-isomaltase co nferred all maltase and isomaltase activity in cedar waxwings. In contrast, all maltase and isomaltase activity in thrushes was necessarily sucrase in dependent, which indicated the presence of maltase-glucoamylase. The absenc e of sucrase-independent maltase activity in cedar waxwings suggests that s ucrase-isomaltase obviates the need for maltase-glucoamylase. Indeed, total maltase and isomaltase activities were much higher in cedar waxwings than in thrushes. Neither waxwings nor thrushes eat starchy foods; sucrase-isoma ltase in waxwings and maltase-glucoamylase in thrushes probably function in digesting glycogen in animal foods. We suggest that digestive traits assoc iated with specialization to monosaccharide-rich fruits (lack of a grinding gizzard) by frugivorous waxwings and thrushes may prevent utilization of s tarchy seeds. Total aminopeptidase-N activity in cedar waxwings was indisti nguishable from the allometric pattern among thrush species, but the distri bution of this enzyme along the intestines of waxwings and thrushes was dis tinctly different, which demonstrates that total enzyme activity can be ins ufficient as a descriptor of the functional activity of brush border enzyme s. Aminopeptidase-N activity peaked in the anterior part of the intestines of thrushes and in the terminal portion of the intestines of waxwings, whic h suggests contrasting strategies for protein digestion from fatty versus s ugary diets, respectively.