Mc. Witmer et Cm. Del Rio, The membrane-bound intestinal enzymes of waxwings and thrushes: Adaptive and functional implications of patterns of enzyme activity, PHYSIOL B Z, 74(4), 2001, pp. 584-593
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.