Exopeptidases of the nervous system

Observations on N alpha-deacetylation of model amino acids and peptides: distribution and purification of a specific N-acyl amino acid releasing enzyme in rat brain

N alpha-Acyl amino acid releasing enzyme (NAARE), an enzyme cleaving acetylMet-Ala at the Met-Ala bond was purified from rat brain cytosol to apparent homogeneity by salt precipitation, gel filtration, and several steps of ion exchange. Levels of NAARE exceeded acylase measured with acetylmethionine in all brain regions and subcellular fractions examined: 60% was associated with cytosol and the remainder with debris or the crude nuclear and mitochondrial-synaptosomal subfractions. Activity was highest in pituitary and was approximately 0.5-0.6 that of liver or kidney. The purified enzyme preferentially hydrolyzed acetylmethionyl peptides: Km for acetylMet-Ala was 0.93; Vmax, 3.5 nmol-1 (kcat, 1185) with pH optimum of 8.9 as compared with 8.2 for acylases measured in cytosol. The purified enzyme was devoid of acylase and common exo- and endopeptidase contamination. Structure-activity relationships examined with synthetic formylated or acetylated peptides indicated no significant effects for di- or tripeptides if the second substituent was Ala, Ser, Asn, or Thr, but the activity was reduced 0.5-fold for Leu, a branched-chain amino acid. No hydrolysis was observed for polypeptides with five or more residues having N-terminal acetylated Tyr (enkephalin) or Ser (alpha-melanocyte-stimulating hormone, thymosin alpha 1), supporting the notion that the enzyme plays a role only in turnover of smaller peptides formed perhaps as a result of endopeptidase cleavage of proteins or polypeptides containing acetylated Met at the N terminus.

Changes in proteolytic enzymes and proteins during maturation of the brain

(1) Changes during development in the levels of proteinases and peptidases were measured in brain homogenates. At all ages di- and tripeptidase levels were 7-15-fold higher than proteinase activity. (2) Cathepsin A and D and neutral proteinase activity first decreased (during the 5 days before birth) and then increased (primarily during the first 10 days after birth) in development. The total enzyme content per unit weight of brain did not change greatly after 10 days, although specific activity fell owing to an increase in protein in older animals. (3) The developmental pattern of activities or peptidases measured with Leu-Gly and Leu-Gly-Gly and of arylamidases measured with Arg- and Arg-Arg-beta-naphthylamides was similar to that of proteinases. Total and specific activities increased rapidly after birth; then total activity did not change and specific activity decreased. (4) The proteinase content of tissue fractions (nuclear and lysosomal-mitochondrial) similarly reached a maximal peak in the rapid growth phase of the brain. (5) The decrease of hydrolytic activity after 10 days of age seems to parallel a decrease in the rates of protein breakdown in vivo, showing parallel behavior with decreasing protein turnover. In contrast, during the first 10 days of life protein turnover and calculated rate of protein breakdown in vivo decrease while the level of hydrolytic enzymes increases.

Pentapeptide (pepstatin) inhibition of brain acid proteinase

The pentapeptide pepstatin obtained from culture filtrates of actinomycetes completely inhibited brain acid proteinase (cathepsin D) at exceedingly low concentrations. Among the brain enzymes tested, the effect is specific for acid proteinase because addition of 1000-fold higher concentrations was without effect on neutral proteinase, aminopeptidase, and arylamidases. Pepstatin also inhibits pepsin as tested with hemoglobin or with N-acetylphenylalanyl-L-diiodotyrosine as substrate. Pepstatin must be regarded as the most powerful agent yet described that inhibits intracellular acidic proteolytic enzyme in brain.