Changes of the quaternary structure of microvillus aminopeptidase in the membrane.

Microvillus aminopeptidase (EC 3.4.11.2) is an enzyme with a molecular weight around 300 000. Normal preparations contain three different subunits (subunit A, Mr 162 000; subunit B, Mr 123 000; subunit C, Mr 61 000). The relationship between the three subunits was studied by immunoelectrophoresis using specific antibodies against individual denatured subunits and by densitometric scanning of polyacrylamide gels after separation of the three subunits. The results suggest that microvillus aminopeptidase initially appears in the membrane as a symmetric molecule built up to two identical A subunits. These subunits are then split into equimolar amounts of subunit B and subunit C by trypsin. Subunit B cannot generate subunit C but may be further degraded. The reaction sequence described is one which occurs in vivo. Treatment of purified aminopeptidase with trypsin increases the specific activity twofold. This phenomenon does not seem to be correlated to the generation of subunit B and subunit C or to the transformation of amphiphilic form into hydrophilic form.