INVIVO SUCRASE-ISOMALTASE AND LACTASE-PHLORHIZIN HYDROLASE TURNOVER IN THE FED ADULT-RAT

We estimated in vivo turnover rates of sucrase-isomaltase and lactase- phlorizin hydrolase in adult rats. Fed animals received a primed conti nuous infusion of phenylalanine (300 muCi, 150 mumol Phe/100 g of body weight for 30 s, then 7.5 muCi, 3.75 mumol Phe/min for 10 to 140 min) . Sucrase-isomaltase and lactase-phlorizin hydrolase were immunoprecip itated from jejunal mucosal membranes; isoforms were separated by SDS- polyacrylamide gel electrophoresis. Endoglycosidase H digestions and ( for lactase-phlorizin hydrolase) N-terminal amino acid sequencing were performed on all isoforms. Specific radioactivity of prosucrase-isoma ltase and prolactase-phlorizin hydrolase isoforms reached isotopic equ ilibrium by 60 and 90 min, respectively. Specific radioactivity of bru sh border sucrase and lactase did not reach steady state. The isotope kinetic, N-terminal amino acid sequencing, and endoglycosidase H diges tion data suggested that one of the high molecular weight lactase isof orms is a dimer of mature lactase. Compartmental modeling of specific radioactivity demonstrated that mean intracellular residence time is 5 9 min for prosucrase-isomaltase isoforms and 68 min for prolactase-phl orizin hydrolase isoforms. Mean residence time in the brush border was 5.8 h for sucrase and 7.8 h for lactase. Fractional synthesis rates w ere 414%/day for sucrase and 307%/day for lactase. Thus, in vivo brush border sucrase and lactase turn over at similar rates in the adult ra t.