L-Methionine 7-amido-4-methylcoumarin

Previous research has shown that the enzymatic activity of hepatic gamma-glutamyltransferase was increased in streptozotocin-induced diabetic rats with no increase in the expression of the protein. The current work has characterized the differences in the kinetic properties of hepatic gamma-glutamyltransferase from diabetic versus control rats. Hepatic gamma-glutamyltransferase was purified from control male and female rats and from rats made diabetic 30 days previously with streptozotocin. The maximal velocity and the Michaelis constant were determined for the purified enzyme with two separate donors (L-gamma-glutamyl-p-nitroanilide or L-gamma-glutamyl-(7-amido-4-methylcoumarin)) in the presence of one of eight acceptors (L-alanine-glycine, L-glycine-glycine, L-methionine, L-glutamate, L-alanine, L-glutamine, L-phenylalanine or L-aspartate). With both donors, hepatic gamma-glutamyltransferase from diabetic rats had a consistently higher kinetic efficiency than gamma-glutamyltransferase from controls. The kinetic efficiency percent increase of diabetic over control gamma-glutamyltransferase when averaged across all acceptors was higher in males than in females. With L-gamma-glutamyl-p-nitroanilide, the kinetic efficiency increase of diabetic over control gamma-glutamyltransferase was higher with poor acceptors than with highly efficient acceptors. These data indicate that there are differences in the physical properties of hepatic gamma-glutamyltransferase from diabetic versus control rats and from female versus male rats.

Aminopeptidases are emerging as exciting novel drug targets and vaccine candidates in parasitic infections. In this study, we describe for the first time an aminopeptidase from three highly pathogenic Leishmania species. Intronless genes encoding a leucyl aminopeptidase (lap) were cloned from Leishmania amazonensis, Leishmania donovani, and Leishmania major, which encoded 60-kDa proteins that displayed homology to leucyl aminopeptidases from Gram-negative bacteria, plants, and mammals. The lap genes were present as a single copy in each genome, and lap mRNA was detected by reverse transcription-PCR in all life-cycle stages of L. amazonensis. Lap assembled into catalytically competent 360-kDa hexamers and demonstrated potent amidolytic activity against synthetic aminopeptidase substrates containing leucine, methionine, and cysteine residues, representing the most restricted substrate specificity of any leucyl aminopeptidase described to date. Optimal activity was observed against L-leucyl-7-amido-4-methylcoumarin (k(cat)/K(m) approximately 63 s(-1) x mm(-1)) with a pH optimum of 8.5. Leishmania Lap activity was inhibited by metal ion chelators and enhanced by divalent manganese, cobalt, and nickel cations, although only zinc was detected in the purified Lap by inductively coupled plasma atomic emission spectroscopy, indicating that zinc is the natural Lap cofactor. Activity was potently inhibited by bestatin and apstatin in a slow binding competitive fashion, with K(i)* values of 3 and 44 nm, respectively. Actinonin was a tight binding competitive inhibitor (K(i) approximately 1 nm), whereas arphamenine A (K(i) approximately 70 microm) and L-leucinol (K(i) approximately 100 microm) were non-tight binding competitive inhibitors. Lap was not secreted by Leishmania in vitro and was localized to the parasite cytosol.

The steady-state kinetics of a full-length and truncated form of the type 2 human methionine aminopeptidase (hMetAP2) were analyzed by continuous monitoring of the amide bond cleavage of various peptide substrates and methionyl analogues of 7-amido-4-methylcoumarin (AMC) and p-nitroaniline (pNA), utilizing new fluorescence-based and absorbance-based assay substrates and a novel coupled-enzyme assay method. The most efficient substrates for hMetAP2 appeared to be peptides of three or more amino acids for which the values of k(cat)/K(m) were approximately 5 x 10(5) M(-1) min(-1). It was found that while the nature of the P1' residue of peptide substrates dictates the substrate specificity in the active site of hMetAP2, the P2' residue appears to play a key role in the kinetics of peptidolysis. The catalytic efficiency of dipeptide substrates was found to be at least 250-fold lower than those of the tripeptides. This substantially diminished catalytic efficiency of hMetAP2 observed with the alternative substrates MetAMC and MetpNA is almost entirely due to the reduction in the turnover rate (k(cat)), suggesting that cleavage of the amide bond is at least partially rate-limiting. The 107 N-terminal residues of hMetAP2 were not required for either the peptidolytic activity of the enzyme or its stability. Steady-state kinetic comparison and thermodynamic analyses of an N-terminally truncated form and full-length enzyme yielded essentially identical kinetic behavior and physical properties. Addition of exogenous Co(II) cation was found to significantly activate the full-length hMetAP2, while Zn(II) cation, on the other hand, was unable to activate hMetAP2 under any concentration that was tested.