CELLULAR BIOCHEMISTRY 



hexosemonophosphate concentration, but this is associated with 

 a minimal increase in the rate of lactic acid formation (11). 

 Instead of envisaging a "barrier," one could also assume that 

 certain muscle enzymes exist as inactive precursors in resting 

 muscle and are changed to active forms during contraction. In 

 fact, epinephrine has such an effect on muscle phosphorylase, 

 which consists in the conversion of an inactive to an active form 

 (see below). For want of a better one, the term "compartment" 

 will be used to describe these deviations from the usual kinetics 

 of enzymes as obtained in cell-free systems, and it will be under- 

 stood that explanations for this phenomenon are highly specu- 

 lative. 



Compartments can also be invoked to explain glycogen 

 formation in the liver. The equilibria of the enzymes involved 

 are such that if the steady-state concentration of inorganic 

 phosphate in contact with phosphorylase were 1 fxM. per gram 

 liver, the steady-state concentration of glucose-6-phosphate 

 would have to be about 7 fxWL for glycogen synthesis to occur. 

 This is a much higher concentration than is found in the liver. 

 Even if one assumes that inorganic phosphate is not in contact 

 with phosphorylase so that the concentration of glucose-6-phos- 

 phate could be lower, one is still faced with the problem of how 

 glucose-6-phosphate formed by hexokinase escapes the action of 

 glucose-6-phosphatase. On the other hand, when glycogen is 

 being broken down, the glucose-6-phosphate formed is accessible 

 to the phosphatase. It is as if glycogen synthesis and degrada- 

 tion were completely separated in the liver cell, in spite of the fact 

 that there is a common intermediate step, catalyzed by phospho- 

 glucomutase. We are at present completely in the dark as to 

 how such cellular operations are accomplished. The examples 

 could be multiplied, since many other intermediates are known 

 which can enter different metabolic pathways. 



To judge from work carried out with isolated mitochon- 

 dria the complexity is very great even on the subcellular level. 

 Factors of permeability are encountered owing to the presence of 

 a membrane and of internal subdivisions as revealed by electron 



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