Studies on the alkali light chains of vertebrate skeletal muscle myosin. Effect of tyrosyl modification on the ability to reassociate to heavy chains

The structures of the alkali light chain subunits A1 and A2 have been studied by examining the effect of the conformationally sensitive reagent tetranitromethane, which reacts specifically with tyrosyl residues. Whereas reaction in the presence of 6 M guanidine hydrochloride results in modification of the three tyrosyl residues of both these light chains, only two tyrosyl residues are exposed to the reagent in the native conformations of these proteins. By gel chromatography of the CNBr-cleaved chains it was demonstrated that the two reactive tyrosyls are those located in the CB-1 and CB-3 segments and that these tyrosyl residues are modified simultaneously and not sequentially. The unreactive tyrosyl residue is in the CB-6 segment and is separated by two residues from the single cysteinyl residue of these chains. It is found that the modified light chains cannot be made to reassociate with the heavy chains by the NH4Cl hybridization procedure of Wagner and Weeds [J. Mol. Biol. 109, 455-470 (1977)] or by the thermal hybridization procedure [Burke and Sivaramakrishnan (1981) Biochemistry 20, 5908-5913]. Furthermore, reduction of the nitrotyrosyl groups to aminotyrosyl residues by sodium dithionite does not restore this effect. The data suggest that regions of the light chains at CB-1 and CB-3 are involved in the association to the heavy chains.

Studies on the interaction of myosin subfragment 1 and immobilized nucleotide. Evidence for different binding domains operating under different solvent conditions

The binding interaction between myosin subfragment 1 isozymes with immobilized nucleotide, where they show differential behavior, has been examined. By employing subfragment 1 hybrids formed by crosses between heavy and alkali light chains, it is possible to demonstrate that the differential behavior is modulated by the alkali light chain component of the protein and not by differences in the heavy chain subunits in these isozymes resulting from the proteolytic treatment used in their formation. The fact that the free alkali light chains show weak differential binding under these conditions suggests that the binding in the case of the subfragment 1 isozymes may occur at a site distinct from the ATPase site. This was substantiated by examining the behavior of subfragment 1 containing [14C] MgADP noncovalently trapped in the ATPase site by the bifunctional reagent N, N-p-phenylenedimaleimide, on agarose-ATP. The data suggest that different ATP binding domains may be operating in myosin depending on the ionic conditions being employed.

Free fatty acids identified as antitryptic factor in soybeans fermented by Rhizopus oligosporus

The trypsin-inhibitory activity observed in cooked soybeans fermented by Rhizopus oligosporus (fungus used in tempeh fermentation) has been examined. The active compounds have now been isolated by ethanol extraction and thin-layer chromatography and have been identified as free fatty acids by infrared spectroscopy and gas-liquid chromatography. Oleic, lineoleic, and linolenic acids are primarily responsible for the increased trypsin-inhibiting activity of cooked soybeans after fermentation. The free fatty acids are liberated from oil in the soybeans by fungal lipase, and they differ from other reported soybean trypsin inhibitors that are protein in nature. Free fatty acids have been previously reported to inhibit various enzymes, such as glycolytic, glyconeogenic, lipogenic, and also proteolytic. Their effect appears to be a nonspecific type of inhibition. Further studies are required to determine their physiological relevance, if any.

Acid protease production by fungi used in soybean food fermentation

Growth conditions for maximum protease production by Rhizopus oligosporus, Mucor dispersus, and Actinomucor elegans, used in Oriental food fermentations, were investigated. Enzyme yields by all three fungi were higher in solid substrate fermentations than in submerged culture. The level of moisture in solid substrate must be at about 50 to 60%. Very little growth of these fungi was noted when the moisture of substrate was below 35%, whereas many fungi including most storage fungi generally grow well on solid substrate with that level of moisture. Among the three substrates tested-wheat bran, wheat, and soybeans-wheat bran was the most satisfactory one for enzyme production. The optimal conditions for maximum enzyme production of the three fungi grown on wheat bran were: R. oligosporus, 50% moisture at 25 C for 3 to 4 days; M. dispersus, 50 to 63% moisture at 25 C for 3 to 4 days; A. elegans, 50 to 63% moisture at 20 C for 3 days. Because these fungi are fast growing and require high moisture for growth and for enzyme synthesis, the danger of contamination by toxin-producing fungi would be minimal.

Milk-clotting activity of proteinases produced by Rhizopus

Rhizopus oligosporus NRRL 3271 produces an enzyme having high milk-clotting activity. High yields of the enzyme were noted in the culture filtrates of milk, wheat flour, or wheat bran. The enzyme was stable at 40 °C, or below, but its activity was destroyed rapidly by heating at 60 °C. The enzyme was fairly stable between pH 3 and 6 and very unstable at a pH below 2 or above 7. The enzyme was recovered from the culture filtrate by ammonium sulfate fractionation (30–75% saturation). When the enzyme preparation so obtained was chromatographed on a DEAE-cellulose column and eluted by salt gradient, four active components were noted, two of which accounted for most of the activity. The NRRL 3271 enzyme and animal rennin behave similarly toward the pH of milk, added calcium chloride, and sodium chloride. Although the NRRL 3271 enzyme caused a higher degree of proteolysis than did rennin, the proteolysis apparently was not high enough to interfere with curd formation. Therefore, the NRRL 3271 enzyme merits further study as a potential replacement for rennin or as an adjunct to be incorporated into rennin for the manufacture of cheese.

Release of proteinase from mycelium of Mucor hiemalis

When Mucor hiemalis NRRL 3103 was grown in soybean medium, only a small fraction of the proteinase produced by the organism appeared in the culture filtrate, whereas the bulk of the enzyme was bound to the mycelial surface. Optimal pH of the proteinase ranged from 3.0 to 3.5. Inclusion of sodium chloride or other ionizable salts in the growth medium, however, resulted in the liberation from the mycelium of the loosely bound enzyme as it was formed. Maximal release of proteinase was achieved at a sodium chloride concentration of 0.5 m. The loosely bound proteinase was eluted also from intact resting mycelium by ionizable salts but not by water or by nonionizable substances. The amount of enzyme eluted from the mycelium depended upon the concentration of sodium chloride up to 0.3 m. Since liberation took place rapidly even at 0 C, a loose ionic linkage must exist rather than a biochemical binding of the enzyme to the mycelium. The recovery of proteolytic activity from repeated salt extractions was greater than that originally detected in the intact mycelium, possibly owing to unmasking of more active enzymes or functional groups. Further proteinase activity was released when salt-extracted mycelium was ruptured. Part of the proteinase thus observed was firmly attached to the cell fraction, and part of it appeared in the supernatant fluid. These conditions implied the presence of intracellular or firmly attached proteinase which could be partially released.

Studies on the extracellular proteolytic enzymes of Rhizopus oligosporus

Two proteolytic enzyme systems were observed in the culture filtrates of Rhizopus oligosporus. One has an optimum pH at 3.0; the other, at 5.5. Both enzyme systems have maximum activities at 50–55 °C and are fairly stable at pH 3.0–6.0. Maximum production of the enzymes occurred after 72 to 96 hours of incubation and then it decreased rapidly. Higher proteolytic activity was noted in the culture filtrates of the organism grown in wheat flour medium than in soybean flour. Data suggest that formation of the enzyme systems appears to be inhibited by soybean extracts.