intracellular proteinases of the hydrolase class that catalyze the hydrolysis of the peptide bond in peptides and proteins.

Cathepsins are widely distributed in animal and plant tissues and in microorganisms. Cathepsins are divided according to the character and specificity of enzymic action into endopeptidases, which are capable of hydrolyzing internal peptide bonds in the molecules of proteins and peptides, and exopeptidases, which act solely on compounds with one or more free terminal carboxyl or amino groups. Cathepsins can also catalyze reactions that may result in lengthening the peptide chain. Almost all cathepsins are activated by compounds containing sulfhydryl groups (cysteine, glutathione).