Leucine aminopeptidase from bovine lens and hog kidney. Comparison using immunological techniques, electron microscopy, and X-ray diffraction.

Abstract

The crystallization of leucine aminopeptidase from hog kidney is reported for the first time. The crystals which diffract to 4-A resolution have the space group P2(1)2(1)2(1) (a = 186.3 A, b = 223.2 A, and c = 80.5 A) and contain four hexamers per unit cell, or one per asymmetric unit. Electron micrographic images of hog kidney leucine aminopeptidase are indistinguishable from micrographs of beef leucine aminopeptidase taken under the same conditions (10). These reveal an equilateral triangle of about 85 A per side, seemingly made of three 40-A diameter spheres. This triangle is circumscribed by another concentric, less-dense triangle of 120 A per side which is rotated 60 degrees with respect to the inner triangle. Immunodiffusion and microcomplement fixation assays indicate that the two enzymes share greater than 90% amino acid sequence homology. This similarity is corroborated by peptide maps of tryptic fragments of the radioiodinated enzymes. The model of the quaternary structure proposed to explain the appearance of electron micrographs of single molecule and crystalline bovine lens enzyme also describes the hog kidney enzyme equally well. That the model of leucine aminopeptidase originally proposed for the beef enzyme also can be used to describe hog kidney leucine aminopeptidase crystal packing in the highly anisometric unit cell provides further corroboration that leucine aminopeptidase in these two species is a hexamer based on two trimers each made of three bilobal promoters.