MITOCHONDRIAL ARGININE KINASE IN THE MIDGUT OF THE TOBACCO HORNWORM (MANDUCA-SEXTA)

Mitochondria isolated from the posterior midgut of the tobacco hornwor m (Manduca sexta) contain arginine kinase. The distribution of mitocho ndrial and cytoplasmic marker enzymes indicates that the presence of m itochondrial arginine kinase is not due to cytoplasmic contamination. Arginine is not oxidized by the midgut mitochondria but, when metaboli c substrates and ATP are present, respiration can be initiated by the addition of arginine. Under these conditions, there is no return to St ate 4 respiration, indicating regeneration of ADP by the arginine kina se reaction. Respiration can be blocked, however, by atractyloside, an inhibitor of the adenine nucleotide translocator, These results indic ate that arginine kinase resides outside the matrix. Mitochondrial arg inine kinase is specific to L-arginine since analogs of L-arginine are ineffective in stimulating respiration in the presence of ATP. Coupli ng between the adenine nucleotide translocator and arginine kinase was investigated using kinetic and thermodynamic approaches, There were n o differences in the activities of arginine kinase in respiring and no n-respiring mitochondria when they were measured at different ATP or a rginine concentrations. This result indicates that arginine kinase doe s not have preferential access to the ATP exported out of the matix. A comparison of the apparent equilibrium constant and the mass action r atio of the arginine kinase reaction also confirms that there is no mi crocompartmentation of the reaction.