Identification of inhibitors of urinary alanine aminopeptidase

Development and in vitro evaluation of systems to protect peptide drugs from aminopeptidase N

PURPOSE

Develop and evaluate systems to prevent aminopeptidase N caused enzymatic degradation of perorally administrated peptide drugs.

METHODS

Bacitracin was covalently bound to the unabsorbable carrier matrix poly(acrylic acid) (paa) in order to avoid any dilution effects of the inhibitor in the intestine as well as systemic toxic side effects. The inhibitory effect of this conjugate, of neutralized paa and N-acetylcysteine was evaluated using a brush border membrane model.

RESULTS

Whereas within 6 h of incubation 65.3 +/- 3.7 mumol/l of the substrate (L-leucine p-nitroanilide) was hydrolyzed under our assay conditions, this metabolism was reduced to 44.5 +/- 6.3 mumol/l and 49.0 +/- 8.8 mumol/l (n = 3-5; +/- S.D.) using 1.5% bacitracin-polymer conjugate and 0.5% N-acetylcysteine, respectively. The same amount of bacitracin as immobilized to the polymer exhibited a comparably weaker inhibitory effect. Neutralized paa did not inhibit membrane bound aminopeptidase N. Covering the membrane with a thin mucus layer led to a significantly lowered inhibitory effect of all tested agents.

CONCLUSIONS

The immobilization of enzyme inhibitors to a carrier matrix and the use of N-acetylcysteine as a novel inhibitor are promising strategies in order to overcome the enzymatic barrier caused by membrane bound peptidases. However the use of effective mucolytic agents seems to be a prerequisite.

Effect of sodium valproate on renal cell brush-border enzymes in rats

The effect of sodium valproate (200 mg/kg body weight) on renal cells was investigated during a 10-day trial, by determining the catalytic activities of alanine aminopeptidase, gamma-glutamyltransferase and alkaline phosphatase in the membrane cell brush-border of the renal proximal tubules. Four days after the administration of sodium valproate, a significant increase was observed in the volume of urine (9.2 +/- 4.2 ml/18-h volume; control group, 3.06 +/- 1.8 ml/18-h volume), and in the catalytic activities of gamma-glutamyltransferase (2.69 times that measured in the control group) and alkaline phosphatase (3.02 times that measured in the control group) in urine. After prolonged treatment the urine became alkaline (pH = 10.5 on day 10), thereby excluding alanine aminopeptidase as a useful indicator of renal cell changes following the administration of sodium valproate. The activities of alkaline phosphatase and gamma-glutamyltransferase in isolated vesicles of renal brush-border membrane cells were significantly increased (p < 0.05) and decreased, respectively. On the basis of the results obtained, we believe that the determination of the catalytic activities of gamma-glutamyltransferase and alkaline phosphatase in urine might prove useful for the follow-up of the renal cell state during therapy with sodium valproate.

A method for determining urinary enzyme activities as nephrotoxic indicators in rats

A simple, useful method was developed for detecting drug nephrotoxicity in rats. Rat urine excreted by stimulation of the sacral part of the back was collected in a beaker, and enzymes in 0.2 ml of the urine were partially purified by centrifugal ultrafiltration using an Amicon MPS-1 kit. Activities of N-acetyl-beta-D-glucosaminidase (NAG), alanine aminopeptidase (AAP), gamma-glutamyltranspeptidase (gamma GTP) and lactate dehydrogenase (LDH), and the protein concentration of the enzyme preparation suspended in phosphate-buffered saline were represented as creatinine ratios. A marked increase in these enzyme activities and protein concentration were observed in rats with kidneys damaged by treatment with cephaloridine, HgCl2, cisplatin or gentamicin. The patterns of increase of these indicators differed with the drug, but in general, LDH showed the highest response and NAG, the longest lasting one. These data agreed with the results reported by other researchers using dialyzed 24-hr urine. Thus, we concluded that this method is an efficient one for determining drug nephrotoxicity in rats.

Human aminopeptidases: a review of the literature

The aminopeptidases constitute a group of enzymes with closely related activities. In clinical chemistry the analysis of the aminopeptidases and of their multiple forms in serum has for a long time been hindered by considerable confusion concerning their identification, and by a lack of characterization. This is in part due to the often large, and sometimes overlapping substrate specificities of the aminopeptidases. This paper reviews the biochemical properties of the different aminopeptidases, the specificities of the assays used for their analysis in serum, some aspects of their multiple forms--which are especially known to occur for alanine aminopeptidase (EC 3.4.11.2)--and the importance of the determination of aminopeptidases and their multiple forms in clinical chemistry.