Development and hormonal modulation of postnatal expression of intestinal alkaline phosphatase mRNA species and their encoded isoenzymes
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1Department of Medicine, Louisiana State University Medical Center, Shreveport 71130.
Insights
Postnatal development in rats shows increased duodenal intestinal alkaline phosphatase (IAP) activity, linked to a specific 3.0 kb mRNA and 90 kDa isoenzyme. This enzyme prefers phenyl phosphate (PhP) over beta-glycerophosphate (beta GP).
Area of Science:
- Biochemistry
- Developmental Biology
- Molecular Biology
Background:
- Intestinal alkaline phosphatase (IAP) activity in rats increases postnatally in the duodenum, with a higher preference for phenyl phosphate (PhP) over beta-glycerophosphate (beta GP).
- The underlying molecular mechanisms for these developmental changes in IAP activity and substrate preference remain largely undefined.
Purpose of the Study:
- To investigate the molecular mechanisms responsible for the developmental increase in duodenal IAP activity.
- To analyze IAP mRNA species, isoenzymes, and their correlation with enzyme activity during postnatal development.
Main Methods:
- Measurement of duodenal and jejunal IAP activity and mRNA levels at postnatal days 12, 18, 24, and 32.
- Analysis of IAP mRNA species (3.0 kb and 2.7 kb) and isoenzymes (90 kDa and 65 kDa).
- Hormonal induction studies using cortisone and thyroxine administration.
Main Results:
- Duodenal IAP activity and mRNA content significantly increased by postnatal day 24, coinciding with the appearance of 3.0 kb mRNA and 90 kDa isoenzyme with a high PhP/beta GP ratio.
- While duodenal mRNA levels doubled by day 32, enzyme activity did not further increase, potentially due to enhanced apical release.
- Jejunal IAP activity decreased postnatally, with only 2.7 kb mRNA and 65 kDa isoenzyme present, exhibiting a low PhP/beta GP ratio.
Conclusions:
- The postnatal surge in duodenal IAP activity is attributed to the expression of a 90 kDa isoenzyme encoded by the 3.0 kb mRNA, which prefers PhP.
- A distinct 65 kDa isoenzyme, encoded by 2.7 kb mRNA and preferring beta GP, is expressed in both duodenum and jejunum.
- Hormonal administration (cortisone/thyroxine) can induce the expression of the PhP-preferring isoenzyme and its associated mRNA, suggesting a regulatory role.
Abstract:
In the rat, intestinal alkaline phosphatase (IAP) activity in the duodenum, but not jejunum, increases on day 22-24 after birth and exhibits higher activity hydrolysing phenyl phosphate (PhP) than beta-glycerophosphate (beta GP) [Moog and Yeh (1973) Comp. Biochem. Physiol. 44B, 657-666]. 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 postnatal days 12, 18, 24 and 32. Duodenal IAP activity and mRNA content were identical on postnatal days 12 and 18, but were 7-fold and 3-fold higher on day 24, respectively than on day 18. The increased IAP activity exhibited a high PhP/beta GP ratio and was accompanied by initial appearance 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 32 respectively. In the jejunum, IAP activity decreased postnatally, but mRNA content was unaltered; only the 2.7 kb mRNA and 65 kDa IAP isoenzyme were present. Administration of cortisone or cortisone+thyroxine induced simultaneous appearance of the duodenal 3.0 kb mRNA and 90 kDa isoenzyme with an increased PhP/beta GP ratio. Thus postnatal increase 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 jejunum and is encoded by the 2.7 kb mRNA.