Mitochondrial permeability for alcohols, aldoses, and amino acids

Summary

Mitochondria isolated from potato tubers were placed in solutions containing various alcohols, aldoses, or neutral amino acids. Based on the osmotic responses in the different media, the reflection coefficients and hence the relative permeabilities of the nonelectrolytes could be determined. The reflection coefficients (σ _j 'S) of the potato tuber mitochondria for alcohols became progressively larger as hydroxymethyl groups were added to the molecule,viz. methanol (σ _j =0.07), ethylene glycol (0.25), glycerol (0.44),meso-erythritol (0.71) and adonitol (0.98). This increase in σ _j (decrease in permeativity) with increasing chain length parallels the decreasing lipid-water partition coefficients of the solutes. The reflection coefficients ofd-sorbitol (1.02) and ofd-mannitol (0.99) indicate that these six-carbon polyhydroxy alcohols are relatively impermeant and hence they would be suitable osmotica in which to suspend mitochondria. The σ _j 'S varied from 0.96 to 1.02 forD-ribose,D-xylose,D-lyxose,D-arabinose, α-D-glucose, β-D-glucose,D-galactose,D-mannose, glycine,L-alanine,L-threonine,L-phenylalanine,L-methionine andL-cysteine, indicating that these sugars and amino acids do not readily diffuse across the pair of membranes surrounding potato mitochondria. By contrast, the σ _j 'S of liver mitochondria for glycine and of pea chloroplasts for most of the same aldopentoses and amino acids are close to zero. Thus, different organelles can vary widely in their permeability properties for nonelectrolytes.