The processing of human mitochondrial leucyl-tRNA synthetase had been previously investigated in insect cell. In the present work, the gene encoding human mitochondrial leucyl-tRNA synthetase with the same N-terminus as that processed in the mitochondria of insect cell was cloned and expressed in Escherichia coli. The enzyme was purified by affinity chromatography on Ni-NTA column. About 6 mg of human mitochondrial leucyl-tRNA synthetase was obtained from 1 liter of culture. The specific activity of the purified enzyme is 127.7 units/mg, the highest activity of the reported results; this enzyme has the potential for characterizing the mitochondrial tRNA mutants associated with some human mitochondrion-related neuromuscular disorders. The kinetic constants for three substrates: leucine, ATP, and E coli tRNA(1)(leu) (CAG) in the leucylation reaction are also reported herein. (C) 2003 Elsevier Science (USA). All rights reserved.