A new protease (araujiain h I) was purified to mass spectroscopy homogeneit
y from the latex of Araujia hortorum Fourn. (Asclepiadaceae) fruits by ultr
acentrifugation and ion exchange chromatography. The enzyme has a molecular
mass of 24,031 (mass spectrometry) and an isoelectric point higher than 9.
3. The optimum pH range for casein hydrolysis was 8.0-9.5. The enzyme showe
d remarkable caseinolytic activity at high temperatures, although its therm
al stability decayed rapidly. The proteinase was activated by thiol compoun
ds and inhibited by common thiol-blocking reagents, particularly E-64 and H
gCl2, suggesting the enzyme belongs to the cysteine protease family. The co
ncentration of active sites as determined by titration with E-64 was 3.3 mu
M. When assayed on N-alpha-CBZ-amino acid-p-nitrophenyl eaters, the enzyme
showed higher preference for the glutamine derivative, followed by those o
f alanine, asparagine, glycine, and leucine, in decreasing order. Partial h
omology (36-48%) with other plant cysteine proteinases was observed in an i
nternal fragment obtained by Protease Vs treatment.