LACTASE-PHLORHIZIN HYDROLASE TURNOVER IN-VIVO IN WATER-FED AND COLOSTRUM-FED NEWBORN PIGS

We have estimated the synthesis rates in vivo of precursor and brush-b order (BB) polypeptides of lactase phlorhizin hydrolase (LPH) in newbo rn pigs fed with water or colostrum for 24 h post partum. At the end o f the feeding period, piglets were anaesthetized and infused intraveno usly for 3 h with L-[4-H-3]phenylalanine. Blood and jejunal samples we re collected at timed intervals. The precursor and BB forms of LPH wer e isolated from jejunal mucosa by immunoprecipitation followed by SDS/ PAGE, and their specific radioactivity in Phe determined. The kinetics of precursor and BB LPH labelling were analysed by using a linear com partmental model. Immuno-isolated LPH protein consisted of five polype ptides [high-mannose LPH precursor (proLPH(h)), complex glycosylated L PH precursor (proLPH(c)), intermediate complex glycosylated LPH precur sor (proLPH(i)) and two forms of BB LPH]. The fractional synthesis rat e (K-s) of proLPH(h) and proLPH(c) (approx. 5%/min) were the same in t he two groups but the absolute synthesis rate (in arbitrary units, min (-1)) of proLPH(h) in the colostrum-fed animals was twice that of the water-fed animals. The K-s values of proLPH, polypeptides were signifi cantly different (water-fed, 3.89%/min; colostrum-fed, 1.6%/min), but the absolute synthesis rates did not differ. The K-s of BB LPH was not different between experimental treatment groups (on average 0.037%/mi n). However, the proportion of newly synthesized proLPH(h) processed t o BB LPH was 48% lower in colostrum-fed than in water-fed animals, We conclude that in neonatal pigs, the ingestion of colostrum stimulates the synthesis of proLPH(h) but, at least temporarily, disrupts the pro cessing of proLPH polypeptides to the BB enzyme.