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-border (BB) polypeptides of lactase phlorhizin hydrolase (LPH) in newborn pigs fed with water or colostrum for 24 h post partum. At the end of the feeding period, piglets were anaesthetized and infused intravenously for 3 h with L-[4-H-3]phenylalanine. Blood and jejunal samples were collected at timed intervals. The precursor and BB forms of LPH were 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 compartmental model. Immuno-isolated LPH protein consisted of five polypeptides [high-mannose LPH precursor (proLPH(h)), complex glycosylated LPH precursor (proLPH(c)), intermediate complex glycosylated LPH precursor (proLPH(i)) and two forms of BB LPH]. The fractional synthesis rate (K-s) of proLPH(h) and proLPH(c) (approx. 5%/min) were the same in the 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 significantly 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%/min). However, the proportion of newly synthesized proLPH(h) processed to 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 processing of proLPH polypeptides to the BB enzyme.