Glycolysis, blocking

Figure 17-1. Summary of glycolysis. 0, blocked by anaerobic conditions or by absence of mitochondria containing key respiratory enzymes, eg, as in erythrocytes.

Identification of the energy source for muscle contraction and determination of the order in which the phosphate esters were metabolized was helped by the use of inhibitors. These inhibitors blocked different stages in glycolysis and caused preceding substrates to accumulate in quantities which could greatly exceed those normally present. The compounds were then isolated, identified, and used as specific substrates to identify the enzymes involved in their metabolism. Iodoacetic acid (IAc) was one of the most important inhibitors used to analyze glycolysis. 

Blocks vesicle trafficking along microtubules lAA blocks glycolysis as well... 

Endocytosis has been shown to be dependent on metabolic activity. A block of metabolic activity might also be used to differentiate endocytosis from fusion or cellular association. Either glycolysis or oxidative phosphorylation can be affected, or both. 

Deoxyglucose (2DG) is a nonmetabolizeable carbon source that blocks glycolysis in concentrations around 50 mM (103). 

Sodium fluoride (104) (1-10 mM) inhibits two enzymes of glycolysis the enolase (phosphopyruvate hydratase) and pyruvate kinase. Therefore, aerobic glucose utilization and lactate formation are blocked. 

Figure 21. Secondary metabolism blocks and amino acid derivation. Note that shikimic acid can be derived directly from photosynthesis and glycolysis through the pentose phosphate cycle, or alternatively as a pyruvic acid postcursor.

Figure 22. Pyruvate derivation and acetyi CoA synthesis. Observe that pyruvate, and subsequently the acetyl CoA pathway, has chain roots in the primary metabolism. Pyruvate can also be synthetized by conversion reactions. The secondary acetyl CoA is constructed as a building block on the pyruvate and glycolysis.

Oxidative decarboxylation of pyruvate by pyruvate dehydrogenase complex is an important pathway in tissues with a high oxidative capacity, such as cardiac muscle (Figure 8.24). Pyruvate dehydrogenase irreversibly converts pyruvate, the end product of glycolysis, into acetyl CoA, a major fuel for the tricarboxylic acid cycle (see p. 107) and the building block for fatty acid synthesis (see p. 181). 

Neither inhibitors of glycolysis, nor uncouplers of cellular energy-releasing processes had any significant effect on the rate of vanadate influx. Phosphate, which is readily taken up by the cells inhibited vanadate influx. Sulfate is not accumulated by the cells, and neither sulfate nor chromate appreciably inhibited vanadate influx at concentrations up to 10.9 mM and 200, uM, respectively. Most significantly, inhibitors of anion exchange across the human red cell membrane such as DIDS (4,4 - diisothiocyanostilbene - 2,2 -disulfonic acid) were also found to block vanadate transport into vanadocytes. 

The conversion, by bacterial extracts, of D-oZtro-heptulose 1,7-diphosphate to shikimate, essentially without side reactions, greatly facilitated subsequent study of the intermediate steps in the synthesis. It was shown that the addition of iodoacetate or fluoride completely blocks this conversion. In the presence of iodoacetate, synthesis is restored by the addition of either D-glyceronic acid 3-phosphate or enolpyruvate phosphate. In the presence of fluoride, only enolpyruvate phosphate is able to restore shikimate synthesis. Neither D-fructose 1,6-diphosphate nor pyruvate reverses these inhibitions. These results suggested that the reactions of glycolysis, from triose phosphate to enolpyruvate phosphate (see Fig. 2), are involved in the conversion of D-oZfro-heptulose diphosphate to shikimate. The effect... 

During alcohol metabolism, NAD+ becomes unavailable to the many other vital body processes for which it is needed, including glycolysis, the TCA cycle and the mitochondrial respiratory chain. Without NAD+, the energy pathway is blocked and alternative routes are taken, with serious physical consequences ... 

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