Biosynthesis of intestinal microvillar proteins. The intracellular transport of aminopeptidase N and sucrase-isomaltase occurs at different rates pre-Golgi but at the same rate post-Golgi.

The kinetics of processing and microvillar expression of aminopeptidase N (EC 3.4.11.2) and sucrose alpha-D-glucohydrolase-oligo-1,6-glucosidase (sucrase-isomaltase, EC 3.2.1.48 and EC 3.2.1.10) were compared by labelling of pig small intestinal mucosal explants with [35S]methionine. The conversion from transient (high mannose glycosylated) to mature (complex glycosylated) form was 1.7-times slower for sucrase-isomaltase than for aminopeptidase N, indicating a slower rate of migration from the rough endoplasmic reticulum to the Golgi complex. Likewise, sucrase-isomaltase appeared in the microvillar fraction at a slower rate than aminopeptidase N. The relative pool sizes of mature and transient forms of both enzymes in intracellular membranes (Mg2+-precipitated fraction) were determined to obtain information on the relative time, spent pre- and post-Golgi, respectively, prior to microvillar expression. This ratio was 0.24 +/- 0.06 (mean +/- SD) for sucrase-isomaltase as compared to 0.40 +/- 0.04 (mean +/- SD) for aminopeptidase N. Considering the slower rate of pre-Golgi transport for sucrase-isomaltase, this indicates that the two microvillar enzymes have rather similar if not identical rates of post-Golgi transport.