摘要:

ABSTRACTData obtained from initial velocity, product inhibition and dead‐end inhibition patterns indicate that the pyrophosphate‐dependent phosphofructokinase from pineapple leaves has a rapid equilibrium random mechanism with E:F6P:MgPPiand E:MgPPi:FBP dead‐end complexes. The pH dependence of enzyme‐reactant dissociation constants suggests that phosphates of F6P (fructose 6‐phosphate), FBP (fructose 1,6‐bisphosphate), Piand PPimust be completely ionized and the lysine is present in the vicinity of the 1‐ and 6‐phosphates of the sugar phosphate and bisphosphate. The pH dependence of kinetic parameters suggests that the enzyme catalyzes the reaction via general acid‐base catalysis with the use of a proton shuttle. The base is required to be unprotonated in both re

ISSN:0145-8884    

DOI:10.1111/j.1745-4514.1993.tb00862.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTCell wall compositions and the levels of ten enzymes of potatoes, jicamas and Chinese water chestnuts were determined to explain the differences in thermal softening of these vegetables. Cell walls were prepared from the tissues by sieving of ethanolic homogenates to remove most of the starch and analyzed for neutral sugars and uronic acids. Jicamas and water chestnuts contain less galactose but more arabinose and glucuronic acid and much more xylose than potatoes. Water chestnuts contain less galacturonic acid than jicamas and potatoes and this was confirmed by extraction of pectin under various conditions. The results suggest that resistance to thermal softening may be related to cell wall polysaccharides rich in arabinose, glucuronic acid and xylose rather than to pectic polysaccharides. The major differences in enzyme compositions are much higher β‐amylase and exopolygalacturonase in jicamas and water chestnuts than in potatoes. Water chestnut exopolygalacturonase is unusually heat‐stable, but the relationship of this enzyme or of β‐amylase to thermal softening is no

ISSN:0145-8884    

DOI:10.1111/j.1745-4514.1993.tb00863.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTProteinases (EC 3.4.21‐24) in langostilla extract and crayfish hepatopancreas were classified using site specific effectors or chelators for either serine, cysteine, aspartic or metallo classes. Azocasein hydrolysis by langostilla and crayfish preparations was inhibited 50% and 40%, respectively, when assayed in the presence of serine proteinase inhibitor, phenylmethylsulfonyl fluoride. A similar degree of inhibition was observed with a trypsin inhibitor. However, no inhibition occurred with a chymotrypsin inhibitor. Less inhibition of azocasein hydrolysis, i.e., 20% and 14%, respectively, resulted with 1,10 phenanthroline. Inhibitors for cysteine and aspartic proteinases did not reduce the azocasein hydrolysis activities significantly. The amidase activity, with N‐benzoyl‐DL‐Arg‐p‐nitroanilide as substrate was more sensitive, in both extracts, to serine proteinase inhibitors than the azocasein hydrolase activity. Results showed the presence of serine proteinases, i.e., trypsin but not chymotrypsin, as the major component and some minor activity of metallo dependent proteinases in the decapods extracts. Zymograms obtained after SDS‐PAGE showed a dozen proteinases in each extract. Some of them were inhibited by a serine proteinase inhibitor and two to three were inhibited by 1,10 phenanthroline, supporting the results of the test tube proteinase activity assays. Furthermore, the reported technique for zymograms allowed direct comparison between extracts and provided information about their composition and the molecular weight of the tar

ISSN:0145-8884    

DOI:10.1111/j.1745-4514.1993.tb00864.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTThein vivosites of melanogenesis in grass shrimps (black tiger shrimp, Penaeus monodonF.), unirradiated and irradiated (5 kGy), was investigated by determining the location of phenoloxidase in heptopancreas, muscle and epidermis using an immunocytochemical technique with electron microscopy. Specific labelling by gold‐IgG complex particles was observed over the microvillar border, apical surface and endoplasmic reticula in hepatopancreas, sarcoplasmic reticula in muscle and chromatophores in epidermis. These results suggest that phenoloxidase is synthesized in R‐cells of heptopancreas, or chromatophores in epidermis and transferred to muscle. There was no difference in the location of phenoloxidase in shrimp tissues that were nonirradiated and irradiated. Consequently, R‐cells in heptopancreas and chromatophores in the epidermis were considered to be important in studying melanogenesis in sh

ISSN:0145-8884    

DOI:10.1111/j.1745-4514.1993.tb00865.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

ABSTRACTNonpasteurized‐refrigerated cucumber pickles were treated with oxidants and reductants to determine their effects on development of translucence (cure) appearance and changes in cell wall pectic substances and protein of mesocarp tissue. Similar responses were obtained for different oxidants that inhibited curing and for different reductants that accelerated curing. Calcium did not affect curing when applied alone or in combination with the redox agents. Levels of pectic substance fractions were not altered in cell walls from mesocarp tissue cured by sodium metabisulfite (SMBS), and only minor changes occurred in cell walls from mesocarp tissue inhibited from curing by hydrogen peroxide. Levels of protein and sulfhydryls were altered by the redox treatments. Peroxide protected against loss in protein content and sulfhydryl levels were substantially lowered. The reductant SMBS enhanced the amount of sulfhydryls and reduced disulfide groups.Therefore, changes in protein characteristics appear to be related to curing of pickle mesocarp tissue. Further studies on curing will include redox agents that were found to regulate the curing proces

ISSN:0145-8884    

DOI:10.1111/j.1745-4514.1993.tb00866.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY