186 A SYMPOSIUM ON RESPIRATORY ENZYMES 



later shown by Pillai (17), and confirmed by us and recently also by 

 Parnas (18), that there is apparently a second mechanism for the 

 dephosphorylation of phosphopyruvate. This conclusion is based on 

 the observation that dialyzed and aged muscle extract or an acetone 

 powder of muscle extract which is unable to split adenosinetriphos- 

 phate, and thus to regenerate adenylic acid, can still split phospho- 

 pyruvate when a catalytic amount of adenosinetriphosphate is added. 

 That one is dealing with a different type of reaction is shown by the 

 fact that in such extracts adenylic acid cannot replace adenosinetri- 

 phosphate. 



Another reaction that leads to the regeneration of inorganic phos- 

 phate is the splitting of adenosinetriphosphate by adenylpyrophos- 

 phatase. There is reason to believe that the activity of this enzyme 

 is increased during muscular contraction (19). Adenylpyrophospha- 

 tase, which is found in most tissues, plays an important regulatory 

 function; by converting adenosinetriphosphate to adenylic acid it 

 can overcome the "bottleneck" which is created in the phosphate 

 cycle by a lack of phosphate acceptors. In addition, it is possible 

 that the reaction described by Pillai, the direct dephosphorylation 

 of phosphopyruvate, plays a physiological role. 



In some experiments with tissue slices the phosphate cycle is so 

 perfectly adjusted that the concentration of inorganic phosphate re- 

 mains virtually unchanged, and this has given rise to the erroneous 

 assumption that one is dealing with a non-phosphorylating glycol- 

 ysis. Dr. Ochoa in our laboratory has recently investigated the 

 glycolysis in brain, which has been regarded by some workers as a 

 tissue with non-phosphorylating glycolysis. In the past most of the 

 work was done with brain slices or brei because with brain extracts 

 the formation of lactic acid was veiy feeble. Geiger (20) made the 

 significant observation that when a brain extract which forms little 

 lactic acid is diluted, a rapid lactic acid formation sets in. This is 

 due to the fact that an inhibitor is present in brain extract, the effect 

 of which is nullified by dilution. Ochoa (21), who confirmed Geiger's 

 observation, was able to show that all the reactions which are 

 characteristic for phosphorylating glycolysis occur in this dilute 

 brain extract. One illustrative experiment is shown in Table 8. The 

 amount of lactic acid formed for an extract corresponding to only 

 40 mg. of tissue is quite large, as good as or better than is obtained 

 with other tissue extracts. Glucose, hexosemono-, and hexosediphos- 

 phate form about equal amounts of lactic acid and the changes in 

 lactic acid and inorganic phosphate correspond to the equations 



