Cathepsin B was the most active cysteine proteinase in the Pacific whiting (Merluccius productus) fish fillet, and cathepsin L in surimi when the activities of the most active cysteine proteinases (cathepsin L, B, and H) were compared. Cathepsin L showed maximum activity at 55°C in both fish fillet and surimi, indicating its function in myosin degradation during conventional cooking of fish fillet and surimi. Washing during surimi processing removed cathepsin B and H but not cathepsin L. Autolytic analysis of surimi proteins showed that the myosin was the primary target, while actin and myosin light chain showed limited hydrolysis during 2 hr incubation. When purified Pacific whiting proteinase was incubated with various component of fish muscle, proteinase was capable of hydrolyzing purified myofibrils myosin, and native and heat-denatured collagen. The degradation pattern of myofibrils by the proteinase was the same as the autolytic pattern of surimi. Inhibition by the food-grade proteinase inhibitors varied with the catalytic type of proteinase. Beef plasma protein (BPP) had a higher percentage of papain inhibitors, followed by whey protein concentrate (WPC), potato powder (PP), and egg white (EW). On the other hand, EW had a higher percentage of trypsin inhibitors followed by BPP, PP, and WPC. EW inhibited trypsin activity completely at levels as low as 1%. WPC inhibited the autolytic activity of fresh surimi. Bovine serum albumin (BSA) was not effective as WPC. WPC can be used as an inhibitor for the Pacific whiting surimi, but high concentration is required. A limited number of inhibitory components were found, as the components in food-grade inhibitors were characterized by inhibitory activity staining. Both EW and PP showed more serine proteinase inhibitors than cysteine proteinase inhibitors. PP showed one cysteine inhibitory component while EW did not show any. BSA in both WPC and BPP acts as an nonspecific competitive inhibitor and reduces the enzyme activity. An unidentified high molecular weight protein (HMP) found in WPC, BPP, and BSA functions as an alternative substrate for papain while it functions as true inhibitor for trypsin. / Graduation date: 1995

Kinetic properties of the two proteases, causing textural degradation of Pacific whiting (Merluccius productus) during heating, were compared and characterized with the synthetic substrate, Z-Phe-Arg-NMec. Pacific whiting P-I and P-II showed the highest specificity on Z-Phe-Arg-NMec, specific substrate for cathepsin L. The K [subscript m] of preactivated P-I and P-II were 62.98 and 76.02 (μM), and k [subscript cat], 2.38 and 1.34 (s⁻¹) against Z-Phe-Arg-NMec at pH 7.0 and 30°C, respectively. Optimum pH stability for preactivated P-I and P-II is between 4.5 and 5.5. Both enzymes showed similar pH-induced preactivation profiles at 30°C. The maximal activity for both enzymes was obtained by preactivating the enzyme at a range of pH 5.5 to 7.5. The highest activation rate for both enzymes was determined at pH 7.5. At pH 5.5, the rate to reach the maximal activity was the slowest, but the activity was stable up to 1 hr. P-I and P-II shared similar temperature profiles at pH 5.5 and pH 7.0 studied. Optimum temperatures at pH 5.5 and 7.0 for both proteases on the same substrate were 55°C. Significant thermal inactivation for both enzymes was shown at 75°C. Preactivated P-I and P-II displayed a similar first order thermal inactivation profile at pH 7.0. At 30 and 90°C, half lives, t [subscript 1/2], for Pacific whiting P-I were 49.50 and 0.20 min and for P-II, 32.54 and 0.18 min, respectively. The rate constant of inactivation for both proteases increased about 200-fold between two limits, 30 and 90°C. Half lives at 55°C, optimum temperature, for P-I and P-II were also determined to be 5.29 and 6.75 min. The increase in thermal inactivation rate constants independent of substrates corresponded to an activation energy for heat denaturation of 21.18 kcal/mol for P-I and 19.97 kcal/mol for P-II by Arrhenius plot. These similar kinetic properties, i.e., kinetic parameters, pH profile and thermal inactivation rate constant, suggested that Pacific whiting P-I and P-II are the same enzyme. / Graduation date: 1994