L-Methionine β-naphthylamide

Lysosome-disrupting enzyme substrates have been used to distinguish between lysosomal and prelysosomal compartments along the endocytic pathway in isolated rat hepatocytes. The cells were incubated for various periods of time with 125I-labelled tyramine cellobiose (125I-TC) covalently coupled to asialoorosomucoid (AOM) (125I-TC-AOM); this molecule is internalized by receptor-mediated endocytosis and degraded in lysosomes, where the degradation products (acid-soluble, radio-labelled short peptides) accumulate, Glycyl-L-phenylalanine 2-naphthylamide (GPN) and methionine O-methyl ester (MOM), which are hydrolysed by lysosomal cathepsin C and a lysosomal esterase respectively, both diffused into hepatocytic lysosomes after electrodisruption of the cells. Intralysosomal accumulation of the hydrolysis products (amino acids) of these substrates caused osmotic lysis of more than 90% of the lysosomes, as measured by the release of acid-soluble radioactivity derived from 125I-TC-AOM degradation. The acid-soluble radioactivity coincided in sucrose-density gradients with a major peak of the lysosomal marker enzyme acid phosphatase at 1.18 g/ml; in addition a minor, presumably endosomal, acid phosphatase peak was observed around 1.14 g/ml. The major peak of acid phosphatase was almost completely released by GPN (and by MOM), while the minor peak seemed unaffected by GPN. Acid-insoluble radioactivity, presumably in endosomes, banded (after 1 h of 125I-TC-AOM uptake) as a major peak at 1.14 and a minor peak at 1.18 g/ml in sucrose gradients, and was not significantly released by GPN. GPN thus appears to be an excellent tool by which to distinguish between endosomes and lysosomes. MOM, on the other hand, released some radioactivity and acid phosphatase from endosomes as well as from lysosomes.

Three aminopeptidases purified from the human placenta were characterized and compared with each other. Aminopeptidase II1 preferred L-arginine- and L-lysine-beta-naphthylamides or p-nitroanilides as substrate, with low or negligible hydrolysis of other amino acid derivatives. It was inhibited by L-arginine, L-lysine and L-methionine. This enzyme activity was highly sensitive to heat treatment, N-ethylmaleimide, p-chloromercuribenzoate, puromycin, bestatin, epsilon-amino-n-caproic acid (EACA) and EDTA. After EDTA, this enzyme could be reactivated by Co2+. It is concluded that aminopeptidase II1 is identical with arginine aminopeptidase (EC 3.4.11.6) from other mammalian tissues. Aminopeptidase II2 preferred L-alanine-beta-naphthylamide and p-nitroanilide as substrates. It was also able to hydrolyse L-leucine, L-arginine, L-methionine and L-lysine derivatives but only very weakly L-cystine and Bz-L-cysteine substrates. This enzyme was inhibited by L-arginine, L-alanine, L-lysine and most strongly by L-leucine and L-methionine. It was resistant to bestatin and heat treatment but sensitive to EACA. EDTA caused a marked suppression, which could be prevented by Co2+ and Zn2+. These characteristics are reminiscent of those of alanine aminopeptidase (EC 3.4.11.-) found in other tissues. The third enzyme was the only one clearly particle bound and was therefore called PB-aminopeptidase. It preferred L-leucine derivatives as substrate but also readily hydrolysed other amino acid-beta-naphthylamides and p-nitroanilides including L-cystine and Bz-L-cysteine substrates. Among the amino acids L-cysteine, L-leucine and L-methionine were inhibitory. Bestatin and thiol reagents were without effect and EACA was only moderately inhibitory. EDTA caused a strong suppression, which could be prevented by Co2+ and Zn2+. These properties are equal to those previously described for the placental cystine aminopeptidase (oxytocinase) (EC 3.4.11.3). All three enzymes had an optimum close to neutral pH but apparent differences in their Km and Vmax values with various substrates. These findings suggest that the three purified aminopeptidases are distinct enzymes. Two of these (aminopeptidases II1 and II2) have not previously been isolated and characterized in the human placenta.

The substrate specificity of rat testicular and epididymal peptidase was investigated using chromogenic substrates, D. L-alanine-, L-arginine-, gamma-N-L-glutamine-, L-leucine-, D. L-methionine-, alpha-N-benzoyl-D. L-arginine-, and N-benzoyl-L-leucine-beta-naphthylamide. The histochemical distribution of peptidase activity demonstrated with these substrates was also investigated in the testis and epididymis. L-Arginine-beta-raphthylamide (Arg-beta-NA) and gamma-N-L-glutamine-beta-naphthylamide (Glu-beta-NA) were mostly hydrolyzed in the testis and epididymis, respectively. Histologically, the activity using Arg-beta-NA as substrate (aminopeptidase B) appeared in both the cytoplasms and nuclei of interstitial cells and spermatogonia and the heads of spermatozoa, while activity using other substrates was found only in the cytoplasms of cells in the germinal epithelium. In the epididymis, strong reaction with Glu-beta-NA (gamma-glutamyl transpeptidase) was found in the apical part of the epithelial cells and in the heads of spermatozoa. Neither alpha-N-benzoyl-L-arginine- nor N-benzoyl-L-leucine-beta-naphthylamide was utilized in either the testis or the epididymis.