Bisulfites, enzyme activation

Bisulfites are oxidised in the body by sulfite oxidase (sulfite ferricytochrom c oxidoreductase) to sulfates, which are excreted in the urine. The enzyme activity is individual, and toxic effects of sulfur dioxide and sulfites are therefore variable. Some individuals tolerate amounts of up to 50 mg/kg, while in sensitive individuals such concentrations will cause headaches, nausea and diarrhoea. In asthmatics receiving steroids, sulfites may cause allergic reactions (ADI = 0.7 mg/kg bodyweight). 

HES is produced from 93—96% dextrose hydrolyzate that has been clarified, carbon-treated, ion-exchanged, and evaporated to 40—50% dry basis. Magnesium is added at a level of 0.5—5 mAf as a cofactor to maintain isomerase stabiUty and to prevent enzyme inhibition by trace amounts of residual calcium. The feed may also be deaerated or treated with sodium bisulfite at a level of 1—2-mAf SO2 to prevent oxidation of the enzyme and a resulting loss in activity. 

Browning can be controlled by adding reducing agents or acidic solutions that inhibit the activity of the PPO enzyme. Solutions of sodium bisulfite, ascorbic acid (vitamin C), and lemon juice are commonly added to freshly cut fruit to retard browning. 

Nitrate and NO2 reduction are rather widespread in SO reducers (Keith and Herbert, 1983 McCready et al., 1983 Mitchell et al., 1986 Maumetal., 1997 Seitz and Cypionka, 1986), and in some cases NOj" is preferred over SO (Seitz and Cypionka, 1986). The bisulfite reductase enzyme in SO reducers displays some activity toward NO2, which partially explains the ubiquity of NO2 reduction. However, it appears that specific NO2 reductases are also present (Liu and Peck Jr., 1981). Unlike denitrifying bacteria that reduce NOj" to N2, the end product of NOj" reduction in SO reducers is NH4 (Widdel and... 

In some countries, nicotinic acid is added to white wheat flour and to other cereal products. The complex with menadione bisulfite, which shows the activity of niacin and vitamin K, has been used in animal nutrition. Nicotinamide in combination with ascorbic acid is used as a meat colour stabiliser, but has not found wider application in the meat industry (under anaerobic conditions, metmyoglobin in raw meat is reduced back to myoglobin by enzymes that contain NAD as a cofactor). 

The tyrosinase of grapes is active but unstable at the pH of must (optimum activity at pH 4.75). Temperatures above 55°C or the addition of more than 50 mg of sulfur dioxide per liter are necessary to denature this enzymatic activity. Lower sulfur dioxide concentrations only modify oxidation rates. In fact, the bisulfite ions regenerate the potential enzyme substrates by reducing the quinones formed. Finally, treating must with bentonite reduces the soluble fraction of tyrosinase. 

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