Intestinal lactase in the neonatal rat. Maturational changes in intracellular processing and brush-border degradation.
The mechanism of decline in the catalytic activity of intestinal lactase during neonatal maturation has not been defined, but a shift in the lactase subunit synthesis from an active 130-kDa subunit to an inactive 100-kDa species has now been noted in the adult rat (Quan, R., Santiago, N. A., Tsuboi, K. K., and Gray, G. M. (1990) J. Biol. Chem. 265, 15882-15888). The subunit structure, synthesis, intracellular assembly, and subsequent degradation of lactase from the brush-border surface membrane was examined in 15-day-old pre-weaned and 30-day-old post-weaned intact rats. Lactase was labeled intraintestinally with [35S]methionine, isolated from Triton-solubilized membranes with monospecific polyclonal anti-lactase, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The protein-stained gel revealed subunits of 225 and 130 kDa, the latter species predominating in both the pre- and post-weaned state. The distinct adult-type 100-kDa moiety was present in post-weaned animals while only a trace of a slightly larger (approximately 110 kDa) species was observed in pre-weaned animals. Quantitation of radioactivity in newly synthesized lactase revealed an increasing prominence of the 100-kDa species in post-weaned rats (130/100 incorporation ratio: pre-weaned 6.2; post-weaned 3.3). Accumulation of newly labeled lactase in brush-border membranes after intraperitoneal [35S]methionine labeling was similar in both groups at 3 h. Despite these comparable rates of lactase synthesis, assembly and insertion in the pre- and post-weaned state, subsequent removal of the 130-kDa unit was more rapid in post-weaned animals (t1/2 = 11 h; pre-weaned t1/2 = 37 h). In intact rats, the neonatal maturational decline in lactase catalytic activities involves both a shift to production of the inactive 100-kDa subunit and increased membrane surface degradation of the active 130-kDa subunit.
Castillo
R O
RO
Department of Pediatrics, Stanford University School of Medicine, California 94305.
Reisenauer
A M
AM
Kwong
L K
LK
Tsuboi
K K
KK
Quan
R
R
Gray
G M
GM
D000375:Aging; D000818:Animals; D000831:Animals, Newborn; D005696:Galactosidases / Q000097:blood*; D007421:Intestine, Small / Q000201:enzymology* / Q000254:growth & development; D007700:Kinetics; D043322:Lactase; D046911:Macromolecular Substances; D008715:Methionine / Q000378:metabolism; D008871:Microvilli / Q000201:enzymology*; D008970:Molecular Weight; D024510:Muscle Development; D009130:Muscle, Smooth / Q000201:enzymology / Q000254:growth & development; D011499:Protein Processing, Post-Translational; D051381:Rats; D011919:Rats, Inbred Strains; D001616:beta-Galactosidase / Q000097:blood* / Q000235:genetics / Q000378:metabolism
D046911:Macromolecular Substances; D008715:Methionine; D005696:Galactosidases; D043322:Lactase; D001616:beta-Galactosidase