DEFECTIVE INTRACELLULAR PROCESSING OF LACTASE-PHLORHIZIN HYDROLASE PROTEIN IN RATS PRENATALLY EXPOSED TO ETHANOL

We have previously shown that fetal exposure to ethanol in rats produc es both structural and biochemical abnormalities in absorptive enteroc ytes. Among the indicators of injury are derangements in the expressio n of lactase-phlorizin hydrolase (LPH), which is an essential enzyme f or the assimilation of milk. In an animal model of fetal alcohol syndr ome, unsuckled newborn rats prenatally exposed to maternal ethanol rev ealed a 10- to 15-fold increase in the number of LPH mRNA molecules pe r absorptive enterocyte, compared with controls (Estrada et al., Alcoh ol. Clin. Exp. Res. 20:1662-1668, 1996). However, lactase activity per cell was similar in both groups. The aim of this study was to charact erize the effect of prenatal exposure to ethanol on the processing of LPH mRNA and protein. RNase protection assays using 3'- and 5'-directe d antisense RNA probes revealed that the LPH mRNA from ethanol-exposed pups is full length. However, metabolic labeling, followed by immunop recipitation using an anti-LPH monoclonal antibody, demonstrated a sig nificant alteration in LPH protein processing. Intestinal explants fro m 21-day ethanol-exposed fetuses that were chased 30 min after a [S-35 ]-methionine pulse showed greater amounts of newly synthesized LPH pre cursors (205 and 220 kDa) and low molecular weight degradation product s than controls. However, despite the increases in LPH precursor, the amount of 130 kDa mature LPH was similar in ethanol-exposed and contro l explants. These data suggest an increase in intracellular degradatio n of LPH precursor in rats prenatally exposed to ethanol, which occurs before its insertion into the microvillus membrane. Biosynthesis of L PH appears to be upregulated at the transcriptional level, which overc omes the degradation of LPH precursor during processing.