PHOSPHORUS METABOLISM 



In considering phosphorus metabolism in fish, attention will be 

 focused on the occurrence of acid-soluble phosphate esters on enzymes 

 such as phosphatases and phosphorylases, and on the process of oxidative 

 phosphorylation. Since phosphorus is of primary importance in carbo- 

 tydrate metabolism and in energy production and storage through the forma- 

 tion of acid-soluble esters, such as hexosephosphate, adenylic acid, and 

 creatine phosphate, it is important to learn if these compounds of phos- 

 phorus occur in fish. Considering the overall intermediary metabolism of 

 energy production in animals, glycolysis yields one-fourth of the energy 

 made available by oxidation of glucose to carbon dioxide and water, and 

 the rest is trapped in the TCA cycle and accompanying terminal oxidative 

 pathways. The on3y means known by which this energy may be trapped is by 

 phosphorylation reactions resulting in the formation of ATP. Since the 

 site for nearly all such phosphorylation reactions is along the electron- 

 oxygen transfer system that transfers hydrogen or electrons from substrate 

 to molecular oxygen, it is most important to examine the evidence for the 

 occurrence of oxidative phosphorylation in marine animals. Also, since 

 some esterification of inorganic phosphates can take place on a substrate 

 level, in which case oxygen is not required, evidence for the occurrence 

 of this reaction is also of considerable importance. One example of this 

 type of reaction is found in the TCA cycle, where 1 mole of ADP is ester- 

 ified when 1 mole of succinic acid is formed from succinyl coenzyme A 

 breakdowno 



Indirect evidence for the occurrence of oxidative phosphorylation 

 in developing fish eggs ( Oryzias latipes) was reported by Ishida (1951). 

 Inhibition of growth and development with a simultaneous increase of 

 oxygen consumption occurred upon treatment with 2,U-dinitrophenol (DNP). 

 Since this compound is a powerful decoupling agent of oxidative phospho- 

 rylation in other animals, it may be assumed that phosphoryation is an 

 essential process for the activity of developing fish eggs. Later, oxi- 

 dative phosphorylation was actually measured in homogenized eggs of devel- 

 oping Oryzias latipes . Complete inhibition of phosphorylation but not 

 succinate oxTdation with DNP addition was observed (Hishida and Makano 

 195U). 



Oxidative phosphorylation by a cell-free particulate system pre- 

 pared from unfertilized eggs of the sea urchin ( Arbacia ) has been shown to 

 be greatly stimulated by the addition of the TCA-cycle intermediates, 

 c-ketoglutarate, oxalacetate and succinate. Apparently the generation 

 of high-energy phosphate bonds in Arbacia eggs is coupled with the func- 

 tioning of the TCA cycle as is the case in mammalian liver or kidney 

 (Keltch et al. 1950). There is also evidence, however, of a second phos- 

 phorylating mechanism in unfertilized Arbacia eggs in addition to the 

 phosphorylation coupled to the oxidation or I'CA-cycle intermediates 

 (Clowes et al. 19£L and Keltch et al. 1951). The uptake of phosphorus 

 in cell-free homogenates with a-ketoglutarate or pyruvate as substrate 

 is completely suppressed by 10-5m dinitrocresol (DNC). But with hexose 

 diphosphate as a substrate, phosphorylation is 63 percent resistant to 

 DNC addition. Indications are that DNC-resistant phosphorylation is 



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