Intestinal amino-oligopeptidase (AOP) is an essential brush-border hydrolytic enzyme required for the surface digestion of nutrient oligopeptides produced from luminal pancreatic protease action on dietary protein. There is an abrupt rise in AOP catalytic activity during postnatal rat development, but the mechanism has not been defined. AOP expression was examined in rats 11 to 60 days of age by measurement of AOP mRNA, catalytic activity, and total AOP protein (by quantitative rocket immunoassay). Specific catalytic activity began increasing at 18 days, achieved a maximum by 22 days (+125% over 11 days), and remained stable thereafter. A 1.1-kb AOP cDNA, generated by the polymerase chain reaction and used to quantify specific mRNA, identified a single 3.8-kb species at all ages on Northern blots. The abundance of beta-actin mRNA, which increased slightly (+40%), and 7S RNA, which did not change, was also measured as developmental controls. The AOP mRNA-to-7S RNA ratio increased dramatically (+410%) between 11 and 60 days of age. A comparable initial rise in AOP activity (+130%) and in its mRNA (+170%) was observed between 11 and 22 days, followed by a divergence of the two curves, with a marked relative excess of mRNA compared with catalytic activity in the 60-day-old adult. The ratio of catalytically active to total immunoreactive AOP protein was higher in 60-day-old adults compared with both 11- to 15-day-old preweaned (65% of 60-day value) and 22- to 30-day-old postweaned (61% of 60-day value) animals.(ABSTRACT TRUNCATED AT 250 WORDS)