DEVELOPMENTAL-CHANGES IN LACTASE-PHLORHIZIN HYDROLASE PRECURSOR ISOFORMS IN THE RAT

The purpose of this study was to determine whether the developmental decline in lactase specific activity (mumol/min/g protein) in the rat was associated with (a) changes in the relative quantities of immunoisolated precursor and mature forms of the enzyme purified by SDS-PAGE and/or (b) immunohistologic changes in the jejunal mucosa. We studied 10- and 16-day-old suckling rat pups, 22-day-old weaned rat pups, and adult female rats (nongravid, pregnant, and lactating). Lactase activity was three- to fourfold higher in 10-day-old pups than in adult rats. Lactase activity was 27% greater in lactating compared with nongravid or pregnant rats. Three molecular forms of the enzyme that migrated identically in all animals were observed on SDS-polyacrylamide gels stained with Coomassie blue: 140-kDa (mature brush border form), 200-kDa, and 220-kDa (apparent precursor forms). There was a striking difference in the proportions of the three polypeptides at different ages that was unrelated to animal status, i.e., pregnant or lactating. As the animals aged, the relative amount of the 140-kDa band declined from 86 +/- 1.1% of the total immunoprecipitated lactase in 10-day old suckling pups to 68 +/- 0.79% in adults. Simultaneously, the relative concentration of the 200-kDa band rose from 1.7 +/- 0.4% in the 10-day-old to 19 +/- 0.6% in adults. The relative concentration of the 220-kDa polypeptide did not change as a function of age. In vivo radiolabel infusion studies using [H-3]phenylalanine demonstrated that the isotope was first incorporated into the 200-kDa band and later into the 140-kDa band. Thyroxine treatment induced a precocious decrease in lactase activity and an increase in the relative concentration of the 200-kDa precursor in the 16-day-old, but not the 10-day-old pup. Immunofluorescent microscopy demonstrated lactase in the brush border membranes of all animals. No staining of the crypt cells was observed in 10-day-old animals. By 16 days of age, faint crypt staining was seen, intensifying in the older animals.