Comparison of leucine aminopeptidase from human lens, beef lens and kidney, and hog lens and kidney

Abstract

Antisera specific for leucine aminopeptidase (LAP) were prepared. Using these antisera in immunodiffusion tests, the identity of LAP isolated from beef lens and kidney is demonstrated. The same pertains to hog lens and kidney LAP. As indicated by only partial fusion of the immunoprecipitates in immunodiffusion plates, leucine aminopeptidases isolated from hog and beef are antigenically similar but not identical. These tests also indicate substantial similarity between a component in human lens homogenates and bovine or hog LAP. Microcomplement fixation tests corroborate these observations and indicate that, under these conditions, LAP from beef lens and kidney, or hog lens and kidney, are indistinguishable. However, there is an approximate 8·5% amino acid sequence difference between beef and hog LAP. Microcomplement fixation tests with human lens homogenate also corroborate immunodiffusion results and indicate an approximate 19% amino acid sequence difference between beef and human LAP. These data establish that LAP is a species-specific enzyme and they indicate that it is not organ specific. Maximal complement fixation occurs at approximately 0·1 μg antigen per tube in those assays in which pure aminopeptidases were tested. This permits standardization of the microcomplement fixation assay for LAP under these conditions. Maximal complement fixation occurs at 160–200 μg human lens homogenate per tube. Assuming that in this quantity of homogenate there is 0·1 μg LAP, then it can be calculated that LAP comprises about 0·05% of the lens protein. This agrees closely with the percentage of LAP in hog and beef lenses. Thus, the reduced LAP activity reported for human lens tissue appears not to result from an absence of the enzyme but rather, may be due to diminished catalytic competency of the enzyme in aged human lens tissue (see Taylor and Juhngen, 1984). The unit evolutionary period, 4·7–5·8 × 10 6 years, indicates that LAP has been highly conserved during evolution.