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Purification and properties of brain cathepsin B. Evidence for cleavage of pituitary lipotropins.

1 
CM-cellulose chromatography of calf brain extracts revealed the presence of several enzymes hydrolysing benzoyiarginyl-β-naphthylamide (Bz-Arg-Nap) one of which resembled cathepsin B (EC 3.4.22.1). The latter was purified to homogeneity and showed an absolute dependence on thiol compounds and was optimally active at pH 6.5. The molecular weight of the purified enzyme was 27000 ± 1500. 
 
2 
Purified enzyme was inhibited by leupeptin, antipain, iodoacetate, Hg2+, zn2+, and partially by chymostatin and phenylmethylsulfonyl fluoride but was unaffected by pepstatin, puromycin, bestatin and bacitracin. The Km with Bz-Arg-Nap was 0.53 mM, Kcat 4.5 min−1. Dipeptidyl and tripeptidyl-acrylamide substrates were hydrolysed also by the brain enzyme but with lower Kcat/Km ratios. 
 
3 
Comparison of the kinetics of inhibition using leupeptin (Ac-l-Leu-Leu-arginal) with Boc-d-Phe-Pro-arginal showed that the latter was non-competitive and the former competitive with Ki of 1.5*10−8 M and 8*10−7 M respectively. 
 
4 
Purified enzyme hydrolysed histones (lysine-rich and type-1), myelin basic protein, porcine β-lipotropin, human β-endorphin and protamine. Hydrolysis of β-lipotropin was accompanied by the generation of a 8000-Mr fragment. 
 
5 
Cathepsin-B-like enzymes were present in highest concentration in rat pituitary, and lower in cerebellum, hypothalamus, medulla oblongata, striatum and spinal cord.