Angiotensin-converting enzyme and a second dipeptidyl carboxypeptidase from hog plasma

A peptidyl dipeptidase-4 from Pseudomonas maltophilia: purification and properties

A peptidyl dipeptidase-4 (bacterial PDP-4) was purified to near homogeneity from a supernatant of Pseudomonas maltophilia extracellular medium. Bacterial PDP-4 is a single-polypeptide-chain enzyme, 82 kDa, with an alkaline isoelectric point. Peptides susceptible to hydrolysis by bacterial PDP-4 include angiotensin 1, bradykinin, enkephalins, atriopeptin 2, and smaller synthetic peptides. N-acylated tripeptides are hydrolyzed, but free tripeptides are not. A free carboxy terminus is required for hydrolysis. Peptides with ultimate and penultimate Pro residues are not hydrolyzed. The enzyme does not require an anion for activity. Bacterial PDP-4 was inhibited by EDTA and the dipeptide Phe-Arg. Thiorphan was an inhibitor only at levels well above those required for inhibition of neutral metalloendopeptidase (NEP), an enzyme for which thiorphan is specific. A second NEP and thermolysin inhibitor, phosphoramidon, did not inhibit bacterial PDP-4. The potent angiotensin-converting enzyme inhibitor lisinopril was not inhibitory. Bacterial PDP-4 is distinguished from a similar enzyme from Escherichia coli, which is not susceptible to EDTA inhibition, and one from Corynebacterium equi, which hydrolyzes free tripeptides. These data indicate that the bacterial PDP-4 catalytic site is unlike those of other enzymes that function either wholly or in part as peptidyl dipeptidases.

The inhibition of acetylcholinesterase by arsenite and fluoride

The effect of fluoride on the rate of reaction of acetylcholinesterase with arsenite, on the rate of dissociation of the enzyme-arsenite complex, and on the equilibrium between enzyme and arsenite was studied. Fluoride decreases the rate of the reaction between acetylcholinesterase and arsenite and changes the apparent equilibrium dissociation constant between the enzyme and arsenite, but even at concentrations as high as 0.2 M has no effect on the rate of dissociation of the enzyme-arsenite complex. The binding of fluoride and arsenite with the enzyme is highly anticooperative and may well be mutually exclusive. These results are consistent with a model in which the binding sites overlap and in which the same functional groups are involved.

Equine angiotensin converting enzyme: a zinc metalloenzyme

1. Angiotensin I converting enzyme from horse plasma has been extensively purified and shown to be homogeneous by disc-gel electrophoresis. 2. The metal ion involved in the catalytic reaction of the enzyme has been identified for the first time as zinc by atomic absorption spectrometry. 3. A number of other physicochemical properties of the enzyme are described and compared with results obtained by other investigators. The molecular weight was determined by gel filtration to be 113 000 daltons. The pH maximum was found to be 7-4. The chloride activation of the enzyme appears to act by facilitation of substrate binding to the enzyme. 4. By use of enzyme inhibitors, tyrosine has been implicated as a functional residue at the active site of the enzyme. 5. The enzyme shows a fairly high degree of specificity towards its substrates.