DEVELOPMENT AND HORMONAL MODULATION OF POSTNATAL EXPRESSION OF INTESTINAL ALKALINE-PHOSPHATASE MESSENGER-RNA SPECIES AND THEIR ENCODED ISOENZYMES

In the rat, intestinal alkaline phosphatase (IAP) activity in the duod enum, but not jejunum, increases on day 22-24 after birth and exhibits higher activity hydrolysing phenyl phosphate (PhP) than beta-glycerop hosphate (beta GP) [Moog and Yeh (1973) Comp. Biochem. Physiol. 44B, 6 57-666]. The mechanism underlying these developmental changes remains unknown. To define possible mechanisms, we have measured IAP activity and mRNA levels, and analysed IAP mRNA species and isoenzymes on postn atal days 12, 18, 24 and 32. Duodenal IAP activity and mRNA content we re identical on postnatal days 12 and 18, but were 7-fold and 3-fold h igher on day 24, respectively than on day 18. The increased IAP activi ty exhibited a high PhP/beta GP ratio and was accompanied by initial a ppearance of the 3.0 kb mRNA and 90 kDa isoenzyme. On day 32, duodenal IAP activity did not increase over the levels on day 24, whereas mRNA levels doubled. The lack of enzyme increase might be related in part to increased apical release, as luminal IAP activity increased from 2% of total mucosal IAP on days 12 and 18 to 7% and 14% on days 24 and 3 2 respectively. In the jejunum, IAP activity decreased postnatally, bu t mRNA content was unaltered; only the 2.7 kb mRNA and 65 kDa IAP isoe nzyme were present. Administration of cortisone or cortisone + thyroxi ne induced simultaneous appearance of the duodenal 3.0 kb mRNA and 90 kDa isoenzyme with an increased PhP/beta GP ratio. Thus postnatal incr ease in duodenal IAP activity is related to the expression of a 90 kDa PhP-preferring isoenzyme encoded by the 3.0 kb mRNA. The low-PhP/beta GP-ratio 65 kDa isoenzyme is expressed in the duodenum and in the jej unum and is encoded by the 2.7 kb mRNA.