Skeletal muscle lactate dehydrogenase isozymes and fibre composition in viral, mycoplasma and bacterial infections in young and old men.

The activities of LD isozymes were studied by thermoinactivation in skeletal muscle biopsies from young and old patients suffering from viral, mycoplasma or bacterial infections and from corresponding controls. The activities of LD2-5 decreased 35-45% and that of LD1 17-29% in the muscle of the patients compared to that of the controls. There were no differences between age groups. In the controls good correlations were demonstrated between the relative activities of the LD isozymes and the muscle fibre composition, but such correlations could not be found in the patients. This indicates that the decrease of the activity of muscle LD in these infections is not associated with alterations in any specific muscle fibre type but rather with an effect of a general reduction of the LD activity.