Desaturation of myristoyl-CoA to myristoleoyl-CoA by hen liver microsomal delta(9)-desaturase.

The desaturation of myristoyl-CoA to myristoleoyl-CoA was measured in microsomal preparations of hen liver. The desaturation was maximal at pH 7.4. The enzymatic activity was linear with time up to 10 min and proportional to microsomal protein concentrations. The initial velocity was linear with substrate concentrations between 13 and up to 200 microM. A decrease in desaturation activity was observed at substrate concentrations greater than 266 microM. There was an absolute requirement for reduced pyridine nucleotide (NADH), while a maximum activity was observed at a myristoyl-CoA:NADH mole ratio of 1. Competitive inhibition studies of myristoyl-CoA desaturation suggest that the inhibitors, stearyl and oleyl-CoA, were more effective than palmitoyl-CoA. Free CoA did not inhibit the delta(9)-desaturase system. The desaturation of myristoyl-CoA was stimulated by bovine serum albumin and reduced by cytoplasmic proteins. The effect of cytoplasmic proteins on the enzymatic reaction was completely abolished by trypsin digestion and boiling for 30 min. On the basis of these data, it was concluded that 9,10-desaturation of acyl-CoA derivatives containing 14-18 carbon fatty acyl chains is catalyzed by the same enzyme.

Direct desaturation of free myristic acid by hen liver microsomal Delta9-desaturase without prior activation to myristoyl-CoA derivative.

Direct desaturation of free myristic acid by hen liver microsomal Delta(9)-desaturase without prior activation to myristoyl-CoA by the addition of adenosine triphosphate (ATP) and CoA was observed when the incubation medium was mixed at mixing speeds of >250 rpm in the presence of fatty acid-binding proteins (FABP). Desaturation was linear with time and proportional to the microsomal protein concentration. Desaturation was maximal at pH 7.9. The greatest desaturation rate was observed at a mixing speed of 500 rpm in the presence of FABP. Desaturation decreased at mixing speeds of >500 rpm. Data suggest that when myristic acid is bound to FABP in the form of protein-monomer complexes, its activation to the CoA derivative is not necessary for it to be desaturated by the Delta(9)-desaturase when using mixing rates of >250 rpm. Myristic acid-FABP complexes serve as substrates for the Delta(9)-desaturase at mixing rates of >250 rpm. Desaturation was reduced by bovine serum albumin and alpha-bromohexadecanoate, and no desaturation was observed in the absence of FABP. These findings suggest that FABP may regulate the accessibility of fatty acids in the desaturation reaction to the active site of the desaturase rather than just protect the membrane-bound desaturase from the cytotoxic effect of free fatty acids.

Antioxidant activity of 3-dehydroshikimic acid in liposomes, emulsions, and bulk oil.

The antioxidant activity of 3-dehydroshikimic acid (DHS), an intermediate in the biosynthesis of aromatic amino acids, was evaluated in three assay systems: bulk oil (lard), liposomes, and a 10% corn oil-in-water emulsion. Upon initiation of peroxidation in the liposome or emulsion systems, DHS exhibited weak antioxidant activity. In contrast, DHS displayed strong antioxidant activity in lard, suppressing peroxidation with activity comparable to that of tert-butylhydroquinone, propyl gallate, and gallic acid and superior to that of alpha-tocopherol. Two major DHS oxidation products, gallic acid and protocatechuic acid, were identified by gas chromatography/mass spectral analysis of lard extracts; both compounds are effective antioxidants in the bulk oil system. In the liposome system, DHS remained intact throughout the assay period. A small amount of gallic acid was observed in extracts of the emulsion; however, protocatechuic acid was not detected. A mechanism to explain the different activities of DHS in the three lipid systems is proposed.

A model system study of the inhibition of heterocyclic aromatic amine formation by organosulfur compounds.

Organosulfur compounds and sodium bisulfite significantly inhibited (P < 0.05) heterocyclic aromatic amine (HAA) formation in model systems containing phenylalanine, creatinine, and glucose. There was, however, no inhibition by the same compounds in a model system containing only phenylalanine and creatinine. Diallyl disulfide (DAD) and dipropyl disulfide (DPD) concentrations in the model systems were significantly decreased (P < 0.05) after heating for 10 min at 180 degrees C. Only very low concentrations of sulfhydryl groups (4.19 and 4.00 micromol) were produced on heating DAD and DPD for 30 min. Reaction of glucose and DAD produced several sulfur-containing compounds. After 10 min of heating at 180 degrees C, HAA formation in the control model systems was increased significantly, and DAD was an effective inhibitor during this heating period. Tetrahydrothiophene-3-one (THT) and tetrahydrothiophene (THP); two products resulting from the interaction of glucose and DAD, had no direct influence on HAA formation in the model systems.

Inhibition of heterocyclic aromatic amine formation in fried ground beef patties by garlic and selected garlic-related sulfur compounds.

The effects of garlic and selected organosulfur compounds (diallyl disulfide, dipropyl disulfide, diallyl sulfide, allyl methyl sulfide, allyl mercaptan, cysteine, and cystine) on the formation of heterocyclic aromatic amines (HAAs) in fried ground beef patties were evaluated. Minced garlic cloves (ca. 4.8 to 16.7%, wt/wt) or organosulfur compounds (0.67 mmol) were added directly to ground beef. Patties (100 g) were fried at 225 degrees C (surface temperature) for 10 min per side. Two patties were fried for each replication, and five replicates were analyzed for each treatment. For each replicate, four subsamples were analyzed (two unspiked subsamples for concentration and two spiked subsamples for the recovery of HAA standards). The volatile sulfur compounds significantly (P < 0.05) reduced concentrations of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine by reductions of 46 to 81%, while average reductions of 35, 22, and 71%, were achieved with cystine, cysteine, and whole garlic, respectively. The volatile sulfur compounds reduced concentrations of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline by 34 to 67%, while reductions of 25, 19, and 63% (P < 0.05) were achieved with cystine, cysteine, and whole garlic, respectively. These studies confirm that garlic and some organosulfur compounds have the potential to reduce HAA formation incooked beef patties.

Estabilidade de alfa e beta carotenos em cenoura desidratada / Stability of alpha- and beta-carotene in dehydrated carrots

O efeito da atividade de água (Aw) na estabilidade de alfa e beta carotenos em cenoura desidratada, armazenada a 21§C na ausência de luz, foi investigado. Foi determinada a isoterma de adsorçäo, a 21§C, da cenoura desidratada. As amostras foram armazenadas em dessecadores sobre soluçöes saturadas de sais, equilibradas para Aw de 0,12 a 0,86. Amostras foram coletadas e analisadas, periodicamente, por HPLC em fase reversa quanto aos teores de alfa e beta carotenos. Verificou-se que a degradaçäo de alfa e beta carotenos na cenoura desidratada, em todas as Aw investigadas, seguia uma reaçäo de primeira ordem. Ao comparar as constantes da velocidade das reaçöes de degradaçäo, observou-se que ao armazenar cenoura desidratada a Aw de 0,23-0,33 e 0,75-0,86, alfa e beta carotenos apresentaram maior estabilidade. Observou-se ainda que alfa e beta carotenos sofrem um mecanismo semelhante de degradaçäo em cenoura desidratada.

Comparaçäo de métodos para a determinaçäo quantitativa de alfa- e beta-carotenos / Comparison of methods for measuring alpha- and beta-carotene concentration

Uma comparaçäo foi feita entre dois métodos em coluna aberta (um deles sendo o método oficial da AOAC) e um método HPLC de fase reversa, na determinaçäo quantitativa do alfa- e beta-carotenos em cenoura desidratada. As amostras foram adicionadas de alfa- e beta-caroteno em duas concentraçöes diferentes e o rendimento foi determinado para extraçäo e cromatografia de coluna. Uma perda substancial de alfa- e beta-carotenos resultou dos dois métodos de coluna aberta. O método da AOAC näo faz distinçäo entre alfa- e beta-carotenos e, portanto, näo pode ser usado para a determinaçäo da atividade de vitamina A. O método HPLC foi o mais rápido, simples e de maior reproduçäo na determinaçäo de alfa - e beta-carotenos e näo requer saponificaçäo.