Abstract
1. The results in three experimental series have suggested that a number of modifying factors must be taken into account in considering the allosteric behaviour of structurally bound intracellular, as distinct from free, haemoglobin. It seems that intracellular haemoglobin maintains its maximal internal stability during oxygen uptake and release by configurational changes which involve not only its acid—base-binding groups but also partial electron-transfer complexes with the stromal macro-molecular framework. Conversely, the affinity of intracellular haemoglobin for oxygen reflects the positive or negative electron pressures prevailing at the haemoglobin-stromal complexing sites.
2. Active glucolysis in intact cells entails continuous poising of metabolic redox potential changes by the stromal material (in the same way as it entails continuous buffering of acid metabolic products). The consequent shifts in stromal redox potential and capacity are transmitted to haemoglobin and are reflected in a changing affinity for oxygen.
3. The behaviour of the intracellular haemoglobin-stromal complex is further modified by extracellular electron donors and acceptors and by the capacity of the cell/extracellular interface to act as an electrochemical barrier between them. It is probably the last variable which is directly influenced by insulin.
4. Two parameters have been introduced as empirical measures of the distinctive allosteric properties of cell-bound haemoglobin: (i) D—pH to represent the apparent cell/extracellular pH gradient; and (ii) D—O2 to represent the volume of oxygen evolved when cells are lysed at a constant PO2. Under a variety of experimental conditions insulin affected both these parameters.
5. The insulin effects were consistent with a concept which rests on three main contentions. (i) Insulin has an immediate physical action on the cell/extracellular interface, altering the redox potential gradient (electron conductance) across the plasma membrane. (ii) Conversely, the formation of an insulin-membrane complex depends, among other factors, on the prevailing trans-membrane redox potential difference. (iii) The consequences of insulin-membrane interaction are both immediate and sustained, and, under appropriate conditions, they are reflected in the affinity of intracellular haemoglobin for oxygen.
6. At a more general level it has been shown that in many respects haemoglobin behaves as a built-in intracellular redox indicator dye. Like most synthetic redox indicators, it is pH dependent.
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Selected References
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