Protein endogenous source

In the human body CO is formed in the catabolism of heme after cleavage of globin (protein part) under catalysis by heme oxygenase (EC 1.14.99.3) with the help of O2 and NADPH. The a-methylene group of heme is quantitatively oxidized to CO. This is the only endogenous source of CO in the human organism. The green tetrapyrrole biliverdin IXa is formed as further reaction product. 

Full arrows primary pathway of exogenous L-Phe predominating at low concentrations hatched arrows L-Phe supply from endogenous sources (biosynthesis and/or protein degradation) open arrows pathway of L-Phe not incorporated into protein. The width of the arrows and the numbers (ng L-Phe/1 culture disc x 30 min.) give an estimate of the maximum capacity of the individual channels. CS I, OS II high affinity and low affinity carrier systems in cell membrane CSy supposed, low affinity carrier system in tonoplast. PP speculative peripheral pool close to cell membrane and in rapid equilibrium with internal pool (IP) feeding protein synthesis. 

L-Phenylalanine in the cytoplasm serves protein and alkaloid biosynthesis. Excess L-phenyl-alanine is accumulated in the vacuoles and can be reused later in alkaloid formation but not in protein biosynthesis. Hence, with respect to alkaloid formation there are two channels for L-phenylalanine a direct, low capacity pathway via the peripheral pool (primary labeling of alkaloids) and an indirect, high capacity pathway from the expandable pool (secondary labeling of the alkaloids). The relative contributions of these two channels vary with the concentration of L-phenylalanine, the time of incubation, etc. Under all experimental conditions, however, in contrast to protein biosynthesis, about 90 % of the L-phenylalanine incorporated into the alkaloids is recruited from endogenous sources, i.e., de novo synthesis and protein degradation. 

It is well known that, oxidative stress describes a condition when the generation of reactive oxygen species (ROS) in a system exceeds the system s ability to neutralize and eliminate them (Sies and Cadenas 1985). The imbalance can resulted from a lack of antioxidant capacity caused by disturbance in production, distribution, or by an overabundance of ROS from endogenous sources or environmental stressors. If not regulated properly, excess ROS can damage cellular lipids, proteins or DNA,... 

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