Identification of the FTBL protein of Sumner and Dounce as a leucine aminopeptidase

The crystalline beef liver protein of Sumner and Dounce (A. L. Dounce, P. Z. Allen, and G. A. Mourtzikos (1978) Arch. Biochem. Biophys. 188 , 251–265 (1)) termed FTBL (football) protein because of the shape of its crystals, has been identified as a crystalline leucine aminopeptidase (LAP), on the basis of its high specific LAP activity and coincidence of its N terminal amino acid sequence (30 amino acids) with that of beef eye lens LAP. Amino acid analyses of the two proteins are also in reasonable agreement when based on the exact monomer molecular weight of beef eye lens protein obtained by the van Loon group ((1982) J. Biol. Chem. 257 , 7077–7081) . Our previously published (1) monomer molecular weight of the FTBL protein was 25% too high, leading to the erroneous conclusion that the beef liver FTBL-LAP protein was a tetramer rather than a hexamer, as found by the van Loon group for beef lens LAP. The present report, taken together with our first paper on the FTBL protein establishes that the FTBL-LAP protein has been isolated from beef kidney and beef spleen as well as from beef liver. We now find that the properties of FTBL-LAP protein indicate that it is the same protein as beef eye lens LAP. The cellular and intracellular distributions of the FTBL-LAP protein have been considered in our first publication on the FTBL protein (1).