In vivo sucrase-isomaltase and lactase-phlorizin hydrolase turnover in the fed adult rat

J Biol Chem. 1993 Jun 25;268(18):13609-16.


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.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Amino Acids / blood
  • Amino Acids / metabolism
  • Animals
  • Food
  • Intestinal Mucosa / metabolism
  • Isoenzymes / metabolism
  • Lactase-Phlorizin Hydrolase / metabolism*
  • Male
  • Microvilli / enzymology
  • Molecular Sequence Data
  • Phenylalanine / metabolism
  • Rats
  • Rats, Sprague-Dawley
  • Sucrase-Isomaltase Complex / metabolism*


  • Amino Acids
  • Isoenzymes
  • Phenylalanine
  • Sucrase-Isomaltase Complex
  • Lactase-Phlorizin Hydrolase