The successive steps in the isolation of mitochondria from chemostat-grown have systematically been investigated for their effects on organelle integrity. Growth rate had a profound effect on the susceptibility of carbon-limited cells towards Zymolyase, whereas the nature of the carbon source had no effect. Stabilization of spheroplasts with at least 2M-sorbitol was required to prevent premature lysis. This was concluded from the amounts of glucose-6-phosphate dehydrogenase liberated during Zymolyase treatments. The influence of the method for disruption of spheroplasts on the quality of the mitochondria was analysed with particular emphasis on respiratory control values and the distribution of marker enzymes among the cell fractions. Disruption by osmotic shock resulted in mitochondria without respiratory control and a high degree of solubilization of NADH and NADPH dehydrogenase activities. Only a gradual decrease of the osmotic value of the medium, preferably by dialysis against a hypotonic buffer, in combination with mechanical disruption with a Potter-Elvehjem homogenizer yielded mitochondria with high respiratory control values and a high retention of NADH dehydrogenase in the organelle. It is concluded that, for the quality of mitochondrial preparations from yeasts, the distribution of NADH dehydrogenase among the cell fractions is a more reliable measure than that of the usual marker enzymes.


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