Lipolytic enzymes and hydrolytic rancidity

Lipolysis, the enzymic hydrolysis of milk lipids to free fatty acids and partial glycerides, is a constant concern to the dairy industry because of the detrimental effcts it can have on the flvor and other properties of milk and milk products. However, free fatty acids also contribute to the desirable flavor of milk and milk products when present at low concentrations and, in some cheeses, when present at high concentrations. The enzymes responsible for the detrimental effects of lipolysis are of two main types: those indigenous to milk, and those of microbial origin. The major indigenous milk enzyme is lipoprotein lipase. It is active on the fat in natural milk fat globules only after their disruption by physical treatments or if certain blood serum lipoproteins are present. The major microbial lipases are produced by psychrotrophic bacteria. Many of these enzymes are heat stable and are particularly significant in stored products. Human milk differs from cows' milk in that it contains two lipases, a lipoprotein lipase and a bile salt-stimulated lipase. The ability of the latter to cause considerable hydrolysis of ingested milk lipids has important nutritional implications

Terpene Specialized Metabolism in Arabidopsis thaliana

Terpenes constitute the largest class of plant secondary (or specialized) metabolites, which are compounds of ecological function in plant defense or the attraction of beneficial organisms. Using biochemical and genetic approaches, nearly all Arabidopsis thaliana (Arabidopsis) enzymes of the core biosynthetic pathways producing the 5-carbon building blocks of terpenes have been characterized and closer insight has been gained into the transcriptional and posttranscriptional/translational mechanisms regulating these pathways. The biochemical function of most prenyltransferases, the downstream enzymes that condense the C5-precursors into central 10-, 15-, and 20-carbon prenyldiphosphate intermediates, has been described, although the function of several isoforms of C20-prenyltranferases is not well understood. Prenyl diphosphates are converted to a variety of C10-, C15-, and C20-terpene products by enzymes of the terpene synthase (TPS) family. Genomic organization of the 32 Arabidopsis TPS genes indicates a species-specific divergence of terpene synthases with tissue- and cell-type specific expression profiles that may have emerged under selection pressures by different organisms. Pseudogenization, differential expression, and subcellular segregation of TPS genes and enzymes contribute to the natural variation of terpene biosynthesis among Arabidopsis accessions (ecotypes) and species. Arabidopsis will remain an important model to investigate the metabolic organization and molecular regulatory networks of terpene specialized metabolism in relation to the biological activities of terpenes

A systematic survey of floral nectaries

The construction of classifications, as well as the understanding of biological diversity, depends upon a careful comparison of attributes of the organisms studied (Stuessy, 1990). It is widely known that data from diverse sources showing differences from taxon to taxon are of systematic significance. Dur-ing the 20th century, systematists have emphasized that their discipline involves a synthesis of all knowledge (Stevens, 1994) or, in other words, the variation of as many relevant characters as possible should be incorporated into the natural system to be constructed. The extent to which particular characters are constant or labile will determine their usefulness to syste-matics. In general, more conservative characters will be valuable in defining families and orders, whereas more labile characters may be useful at the ge-neric and specific levels (Webb, 1984). There is no doubt that floral characters are among the most used in the classification of flowering plants. At the same time, they constitute essential features in diagnostic keys to taxa in both taxonomic treatments and Floras (Cronquist, 1981, 1988).Fil: Bernardello, Gabriel Luis Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; Argentin