Stereoselectivity of lipases in supercritical carbon dioxide. I. Dependence of the regio- and enantioselectivity of porcine pancreas lipase on the water content during the hydrolysis of triolein and its partial glycerides.

The stereoselectivity of porcine pancreas lipase (PPL) was investigated during the enzymatic hydrolysis of triolein and its partial glycerides in the presence of supercritical carbon dioxide (SCCO2) as reaction medium. The water content of the immobilized lipases was varied. The partial glycerides were separated into mono- and diglycerides by TLC, converted to their 3,5-dinitrophenylurethane derivatives and subsequently resolved into sn-1,2 and sn-2,3 enantiomers (estimation of dioleins) or into sn-1 and sn-3 enantiomers (estimation of monooleins) by HPLC on a chiral stationary phase (Sumichiral OA-4100). In all reactions under the conditions employed, PPL revealed a distinct preference for the sn-3 position of the glycerol. However, the stereoselectivity depends on the reaction time, the substances initially used and the enzyme water content. It seems that the effect of the enzyme water content on the activity and selectivity of porcine pancreas lipase in SCCO2 is based on a modification of the "micro-environment' of the enzyme by the solution of CO2 in water, causing a decrease of the pH value.

[Progress report. Lipid peroxidation. 2. Secondary reactions]. / Fortschrittsbericht. Lipidperoxydation. 2. Mitt. Sekundäre Reaktionen.

Lipids and lipid containing foods are altered by autoxidative processes resulting in the formation of hydroperoxides which are considered as the actual primary autoxidation products. These hydroperoxides cause a variety of secondary reactions giving di- and trihydroperoxides, hydroperoxyepidioxides, endoperoxides and hydroperoxyepoxides, so called secondary autoxidation products at a primary level. Sometimes they are only minor substances. The formation of primary autoxidation products is accelerated by reaction conditions as increased temperature, oxygen, metal ions and sensibilizers but is inhibited by natural and synthetic antioxidants. The decomposition of hydroperoxides and secondary autoxidation products at the primary level results in volatile substances decreasing the flavour quality of lipid containing foods. Simultaneously there are other cleavage and decomposition products remaining in the lipid causing a reduction of its oxidative stability.

[Progress report. Lipid peroxidation. 1. Primary reactions]. / Fortschrittsbericht. Lipidperoxydation. 1. Mitt. Primärreaktionen.

Edible fats and oils are an important component of human nutrition. There are many kinds of changes and destruction of lipids. One of the most important processes is the autoxidation of essential fatty acids. Primary products of oxidation are mono-hydroperoxides of fatty acids formed by three different pathways. Thermal activation of fatty acids will be followed by thermodynamically controlled distribution of the hydroperoxide isomers. With enzymatic or photochemical oxidation specific distribution of isomeric hydroperoxides is observed. These products can be also used for the determination of the type of oxidation which took place. The hydroperoxides formed in the process of oxidation cause a radical chain reaction with a powerful progress of oxidation. The products formed by chain termination are characterized by different stability. These products include the possibility of decomposition and consequent reactions.

[Progress report. Lipoxygenases--their significance in lipid chemistry]. / Fortschrittsbericht. Lipoxygenasen--ihre Bedeutung in der Lipidchemie.

Lipoxygenase catalysed reactions play an important role in the field of lipid peroxidation. The enzyme is characterized concerning its sources, biological importance, isolation, substrates, active centre and inhibition. Extensive explanations should show the complex mechanisms of enzyme catalyses divided into two major ways: dioxygenase and hydroperoxidase reactions. Food science specific importance and expected effects on food systems are discussed and related to reaction products generated during catalysis and enzymatic processes both preceding and following lipoxygenase reaction.