Intestinal lactase synthesis during postnatal development in the rat.

To elucidate the mechanism of the developmental decline in intestinal lactase activity at weaning, we examined lactase synthesis in suckling and adult rats. Lactase was purified to homogeneity from pooled intestines of newborn rats and used to raise a monospecific antibody. Using this antibody, we developed a quantitative immunoprecipitation assay for lactase. Intestinal microvillus membrane proteins were labeled in 15-day and adult rats by intraluminal pulse-chase with 3H-leucine, and newly synthesized lactase quantified by immunoprecipitation. When lactase synthesis was expressed as the quantity of microvillus membrane lactase synthesized relative to total microvillus membrane protein synthesized, a significantly greater proportion of 3H-leucine incorporation into lactase was demonstrated in the suckling animals. No structural differences between newly synthesized suckling and adult lactase were observed when they were compared by SDS-polyacrylamide gel electrophoresis and fluorography. These data suggest that a change in the rate of lactase synthesis plays a role in the postweaning decline in enzyme activity.