No abstract

Proteolytic enzymes in the sarcoplasmic fluid of Pacific whiting (Merluccius productus) were studied to determine which proteases might be involved in the severe textural defect associated with this species. The results were compared to those obtained for true cod (Gadus macrocephalus), a fish having firm texture. Two pH optima of enzymic activity were noted for whiting, pH 3.5-3.9 and 7.1-7.2, Two pH optima were also observed for cod, pH 3.2-3.6 and 7.8-8.0. Whiting but not true cod contained enzymic activity similar to cathepsin B. Cathepsin C activity was greater in whiting than in cod.

The distribution of newly absorbed copper among serum proteins obtained from the portal circulation of rats was examined by conventional and high-performance gel filtration chromatography, affinity chromatography, and Western blotting. Within 10-30 min after being administered by gavage or directly into the intestine, 67Cu and 64Cu, respectively, were recovered in the albumin fraction. By 8 h after administration of the radionuclides, virtually all of the radioactivity was found with ceruloplasmin. Affigel blue fractionation and subsequent Superose-6 chromatography further demonstrated that all of the copper in the albumin-containing fractions was in fact bound to this protein rather than high molecular weight moieties. Vascular perfusion of the isolated rat intestine, where 64Cu was infused into the lumen, showed that newly absorbed 64Cu in the vascular perfusate collected from the cannulated portal vein was associated with albumin. Uptake of radioactivity by isolated rat liver parenchymal cells from medium containing rat serum with 67Cu bound to albumin was demonstrated. In vitro binding of 64Cu to serum proteins that were transferred to nitrocellulose by Western blotting techniques showed that albumin is essentially the only protein that binds appreciable amounts of copper. The data suggest that albumin is the plasma protein that is responsible for the initial transport of copper after absorption.

Several sensitive substrates for porcine pancreatic elastase, chymotrypsin, and trypsin were prepared that utilize the permanently charged, fluorogenic cation 1-methyl-6-aminoquinoline (MAQ+) as the leaving group. Kinetic rates for the hydrolysis of substrates were determined fluorimetrically and compared with analogues having 6-aminoquinoline (6-AQ) as an uncharged leaving group. It was found that substrates containing the quaternized leaving group generally have a higher kcat/Km ratio. An exception to this trend was noted with a trypsin substrate, Bz-DL-Arg-MAQ+. During the course of this investigation, several significant advantages of the MAQ+ ion as a fluorogenic leaving group in protease substrates were found: (a) its appearance can be measured fluorimetrically using wavelengths of light that result in its maximal fluorescence, while under these conditions, the unhydrolyzed substrate is essentially nonfluorescent, (b) it confers a high degree of water solubility to hydrophobic peptides, thereby eliminating the need for organic cosolvents to dissolve substrates, and (c) quaternized substrates can be prepared readily and in good yield from the corresponding 6-(peptidylamido)quinolines. These positively charged synthetic fluorogenic substrates are, therefore, useful probes for investigating the steric and electronic properties of the active-site environment of proteolytic enzymes.