Suckling rat colon synthesizes and processes active lactase-phlorizin hydrolase immunologically identical to that from jejunum

Abstract

To identify potential tissue-specific characteristics of intestinal glycoprotein synthesis and processing, rat intestinal lactase-phlorizin hydrolase (L-Ph) was studied after pulse-labeling of colonic explants from 5-d-old suckling rats in organ culture and the data compared to similar studies in rat jejunum. Histologic sections of 5-d-old proximal colon showed villus-like structures lined with columnar epithelial cells. Lactase and phlorizin hydrolase activities showed tissue-specific developmental patterns. Using a MAb to small intestinal L-Ph, we were able to immunoprecipitate from colon at different ages a protein that hydrolyzed lactose and phlorizin, and whose activity was not inhibited by p-chloromercuribenzoate. After pulse-labeling for 60 min and chase for 30 min, immunoprecipitated L-Ph from total homogenates of rat colonic explants appeared on fluorography of SDS-PAGE as one band of approximately 205 kD. With increasing time of chase, it took 240 min before the precursor form was converted to the intermediate form (equivalent to the 180-kD form in jejunum) and the mature form (equivalent to the 130-kD form in jejunum), although these conversions in the jejunum were observed within 60 min of chase, and only 30 min of pulse labeling. When compared on SDS-PAGE to immunoprecipitated jejunal L-Ph, the precursor form in the colon had a slightly higher apparent mol wt than the corresponding precursor form found in the endoplasmic reticulum-Golgi fraction of the jejunum. The intermediate as well as the mature L-Ph forms in the colon were also both somewhat higher in apparent molecular weight than the same bands in the microvillus membrane fraction from jejunal explants. Removal of N-linked oligosaccharides from jejunum and colonic forms of L-Ph produced bands on SDS-PAGE with identical mobility, suggesting that the proteins were the same. The data demonstrate that, in neonatal colon, enzymatically active L-Ph undergoes biosynthetic and processing events similar to those in the jejunum. During early life, colonic L-Ph may function in the salvage of lactose not absorbed in the small intestine.