A predominant, heat-activated proteinase in muscle extract of arrowtooth flounder (Atheresthes stomias) was purified to 55-fold by heat treatment, followed by a series of chromatographic separations. The apparent molecular mass of the purified enzyme was 27 kDa by size exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The proteinase had high affinity and activity toward Z-Phe-Arg-NMec with K(m) and k(cat) values of 8.2 microM and 12.2/s, respectively. Activity was inhibited by sulfhydryl reagents and activated by reducing agents. The purified proteinase displayed optimal activity at pH 5.0-5.5 and 60 degrees C, respectively. Consistent with the properties of proteases from other species, the heat-activated proteinase in arrowtooth flounder can be identified as cathepsin L.