STRUCTURE-ACTIVITY STUDIES ON MAMMALIAN TISSUE LYTIC ENZYMES: CHEMICAL CHARACTERIZATION AND SUBSTRATE SPECIFICITY OF RAT KIDNEY NUCLEAR LYSOZYME*

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Abstract
Studies on the structure and substrate specificity of purified rat kidney nuclear (RKN) lysozyme are reported. The carboxyl and amino terminal residues of RKN-lysozyme were leucine and alanine respectively. The amino acid composition indicated similarities and differences as compared with that of hen egg white (HEW) lysozyme. There were alterations in the 9 amino acid residues, Lys, His, Arg, Asp, Glu, Pro, 1/2 Cys, Tyr and Trp. The other 9 residues were present in identical proportions to those of HEW-lysozyme. The decrease in the arginine and aspartic acid residues was found to be compensated by the increase in the number of lysine, histidine and glutamic acid residues. The overall ratio of the acidic to basic amino acids has thus been conserved in the mammalian enzyme. In addition, RKN-lysozyme contained decreased numbers of Trp, Tyr and 1/2 Cys and increased numbers of proline residues as found in HEW-lysozyme. RKN-lysozyme did not cross react with heterologous antibodies produced against HEW-lysozyme, and vice versa. RKN-lysozyme showed distinct specificity towards the lysis of Micrococcus luteus. Against this substrate, it was 3 times more efficient than HEW-lysozyme. It also cleaved Escherichia coli B, but its efficiency was half as much as with M. luteus. However, it cleaved Pseudomonas septica and Bacillus subtilis at a rate similar to HEW-lysozyme under identical conditions.
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