Degradation of secondary products

Barz, W. and Koster, J. (1981) Turnover and degradation of secondary products, in The Biochemistry of Plants. Vol. 7. Secondary Plant Products (ed. E.E. Conn). Academic Press, Orlando, pp. 35-84. 

Synthesis, storage, and degradation of secondary products may go on simultaneously in different compartments of one cell. This cannot occur without continuous movement in and out of the storage compartments, e.g., the vacuoles of plants. Intracellular compartmentalization is especially important in the regulation of the amount of this type of secondary compounds. In several instances, however, the synthesis of secondary products and their further transformation/ degradation are part of different programs of cell differentiation. In this case biosynthesis and transformation and/or degradation are separated in space and/ or time. 

The primary products from autoxidation are hydroperoxides, which are often simply referred to as peroxides. Peroxides are odorless and colorless, but are labile species that can undergo both enzymatic and nonenzymatic degradation to produce a complex array of secondary products such as aliphatic aldehydes, alcohols, ketones, and hydrocarbons. Many of these secondary oxidation products are odiferous and impart detrimental sensory attributes to the food product in question. Being able to monitor and semi-quantitate the development of peroxides by objective means (e.g., PV determination) over time is important for food scientists who want to characterize the quality of an oil or a lipid-containing food product, even though the peroxides themselves are not directly related to the actual sensory quality of the product tested. 

The understanding of the degradation of natural products such as camphor has been greatly enhanced by understanding the catalytic cycle of the cytochrome P-450 enzyme P-450cam in structural detail.3,4 These enzymes catalyze the addition of 02 to nonactivated hydrocarbons at room temperatures and pressures - a reaction that requires high temperature to proceed in the absence of a catalyst. O-Methyltransferases are central to the secondary metabolic pathway of phenylpropanoid biosynthesis. The structural basis of the diverse substrate specificities of such enzymes has been studied by solving the crystal structures of chalcone O-methyltransferase and isoflavone O-methyltransferase complexed with the reaction products.5 Structures of these and other enzymes are obviously important for the development of biomimetic and thus environmentally more friendly approaches to natural product synthesis. 

The reactions of CF3O radicals are critical toward accessing the potential impact of secondary products resulting from alternative halocarbon degradation on ozone perturbations. The potential for ozone removal by CF3O radicals was recognized by Francisco and Williams [132]. Example cycles originally pointed out are ... 

On the other hand, the pyrograms observed in flash Py-GC at 720°C almost entirely consisted of the fragments formed via homolytic degradations, although many were identical with those observed by Py-FMS. In addition, the difference between Nomex and Kevlar with Py-GC was much less than those observed in Py-FIMS. Moreover, the formation of secondary products, such as biphenyl derivatives in Py-GC, was much less than that in Py-FIMS. The differences between the results by Py-GC and by Py-FIMS could be attributed to the difference in the flnal pyrolysis temperature and the heating rate. 

Suppression of secondary product formation by excess nutrients, especially by glucose and other easily degradable carbon sources, but also by nitrogen-containing compounds and phosphate, is a general phenomenon in microbial cultures. 

For a long time metabolic stability was thought to be a characteristic of secondary products. Recent experiments, however, have demonstrated that many secondary substances are transformed or are even degraded to compounds of primary metabolism. Three types of secondary compounds may be distinguished with respect to metabolic stability (a) the truely metabolically inert end products, (b) the products stable at a given physiological or developmental state, and (c) the substances undergoing continuous turnover. 

The accumulation of products undergoing turnover depends on the rates of synthesis and of transformation/degradation. These may be regulated independently of each other. Since so many endogenous and exogenous factors affect the amount of secondary products stored, it may vary with the developmental state, the season, the climate, and even the time of the day. 

Man, like most vertebrates, for instance, is unable to degrade the aromatic amino acids L-histidine and L-tryptophan, the skeletons of purines and porphyrines, methyl groups etc. After transformation to secondary products the skeletons of these substances and groupings are removed with the urine and in feces and sweat (Table 60). Most plants also cannot degrade certain aromatic amino acids. Hence they store large amounts of secondary products, e.g., derived from L-tryptophan (D 21) and L-phenylalanine/L-tyrosine (D 22). 

Many groups of secondary products undergo synthesis and degradation in the producer organisms (A 5). Transformation and degradation may also play an important role, if toxic secondary products escape into sensitive areas of metabolism. 

Barz W, Koster J (1981) Turnover and degradation of secondary (natural) products. In Stumpf PK, Conn EE (eds) The biochemistry of plants, vol 7. Academic, New York, pp 35-84... 

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