We determined along the small intestine of young and adult rabbits the activities of lactase (LPH) and sucrase (SI), the levels of their cognate mRNAs, and examined the in vitro biosynthesis of LPH and pro-SI. Lactase activity is low in the proximal 1/3 of the intestine, whereas the mRNA levels are high. However, the rates of biosynthesis of the LPH forms correlated well with the steady-state levels of LPH mRNA in all segments, indicating that factor(s) acting post-translationally produce a decline in brush border LPH in the proximal small intestine. These factor(s) are not involved in the processing of pro-LPH to mature LPH, since the relative amounts of the various forms of LPH are almost the same along the small intestine. Unexpectedly, we find that also for SI the ratio of activity to mRNA is low in proximal intestine. The biosynthesis of pro-SI correlates with the steady-state levels of its mRNA. Hence, the steady-state levels of LPH and SI along the small intestine are regulated both by mRNA levels and by posttranslational factor(s).
