In vivo sucrase-isomaltase and lactase-phlorizin 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 continuous infusion of phenylalanine (300 microCi, 150 mumol Phe/100 g of body weight for 30 s, then 7.5 microCi, 3.75 mumol Phe/min for 10 to 140 min). Sucrase-isomaltase and lactase-phlorizin hydrolase were immunoprecipitated 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-isomaltase and prolactase-phlorizin hydrolase isoforms reached isotopic equilibrium by 60 and 90 min, respectively. Specific radioactivity of brush border sucrase and lactase did not reach steady state. The isotope kinetic, N-terminal amino acid sequencing, and endoglycosidase H digestion data suggested that one of the high molecular weight lactase isoforms is a dimer of mature lactase. Compartmental modeling of specific radioactivity demonstrated that mean intracellular residence time is 59 min for prosucrase-isomaltase isoforms and 68 min for prolactase-phlorizin hydrolase isoforms. Mean residence time in the brush border was 5.8 h for sucrase and 7.8 h for lactase. Fractional synthesis rates were 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 rat.
Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030.
M A
MA
Dudley
D L
DL
Hachey
A
A
Quaroni
T W
TW
Hutchens
B L
BL
Nichols
J
J
Rosenberger
J S
JS
Perkinson
G
G
Cook
P J
PJ
Reeds
D000595:Amino Acid Sequence; D000596:Amino Acids / Q000097:blood / Q000378:metabolism; D000818:Animals; D005502:Food; D007413:Intestinal Mucosa / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism; D010694:Lactase-Phlorizin Hydrolase / Q000378:metabolism*; D008297:Male; D008871:Microvilli / Q000201:enzymology; D008969:Molecular Sequence Data; D010649:Phenylalanine / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013394:Sucrase-Isomaltase Complex / Q000378:metabolism*
D000596:Amino Acids; D007527:Isoenzymes; D010649:Phenylalanine; D013394:Sucrase-Isomaltase Complex; D010694:Lactase-Phlorizin Hydrolase