EFFECTS OF MALONATE ON GLUCOSE METABOLISM 129 



cycle. The formation of lactate from glucose in extracts of rat muscle is 

 inhibited over 90% by 60 mM malonate; simultaneously there is a decrease 

 in inorganic phosphate, creatine-P, and readily hydrolyzable phosphate, 

 accompanied by an increase in nonhydrolyzable phosphate (Fawaz and 

 Fawaz, 1962). It was concluded that the acid-resistant phosphate fraction 

 occurring in the presence of malonate must be made up of glycolytic inter- 

 mediates, and it was then shown by analyses of the incubated extracts at 

 various times that there is accumulation of 3-P-glycerate and glycerol- 1-P 

 particularly, with smaller contributions from phosphoenolpyruvate and 

 2-P-glycerate (see accompanying tabulation). Furthermore, the addition 



of these intermediates to a malonate-treated extract resulted in the ap- 

 pearance of a major fraction as 3-P-glycerate, whereas in control incuba- 

 tions they break down to inorganic phosphate and pyruvate or lactate. 

 This might imply a block at the pyruvate kinase and some reversal of the 

 glycolytic pathway which cannot proceed beyond 3-P-glycerate due to the 

 lack of ATP. The addition of pyruvate to an inhibited extract leads mainly 

 to the formation of lactate. Pyruvate kinase from rabbit muscle is inhibited 

 86% by 60 mM malonate and this correlates quite well with the results 

 seen in the extracts. Malonate at 30 mM only partly inhibits glycolysis, 

 causes less accumulation of the phosphorylated intermediates, and inhibits 

 pyruvate kinase 67%. The relative lack of affect of 30 mM malonate on 

 cardiac function is probably due to a combination of two factors: the intra- 

 cellular malonate concentration is undoubtedly less than 30 mM, and it 

 is likely that glycolysis in the heart can be depressed to a certain degree 

 before failure occurs, the myocardium having other sources of energy 

 available.* Glycolysis in dog muscle extracts proceeds somewhat differently 

 than in rat muscle extracts (e.g. accumulation of hexose phosphates occurs) 

 and the response to malonate is consequently different (Fawaz et at., 

 1963). High concentrations of malonate cause the accumulation of the same 



* Since the heart is generally considered to obtain much of its energy from fatty 

 acid oxidation under certain circumstances, it would be important to know the effects 

 of these high concentrations of malonate on this pathway. However, with the cycle 

 inhibited, the generation of energy from the oxidation of fatty acids should be reduced. 



