Slow axoplasmic transport of mitochondria (MAO) and lactic dehydrogenase in mammalian nerve fibers.

The axoplasmic transport of the enzyme monoamine oxidase (MAO) (monoamine:O2 oxidoreductase, (deaminating) EC 1.4.3.4), a marker for mitochondria, and lactic dehydrogenase (LDH) (L-lactate:NAD oxidoreductase, EC 1.1.1927), a soluble component of axoplasm, was studied in cat sciatic nerve. For both these enzymes a linear accumulation was found in the nerve proximal to ligations over a period of at least 20 h. In double-ligation experiments no evidence of a depletion of enzymes within the nerve segment was found over this period of time as would be the case if some portion of the enzymes was carried by fast axoplasmic transport. Both the soluble protein enzyme LDH and the mitochondria, shown by MAO, are thus considered to be moved down the nerve by slow axoplasmic transport. Some differences in the two materials were seen in the greater fall in the level of MAO compared to LDH within the double-ligated segment over the succeeding period from 20 to 48 h. These changes are considered with respect to the transport filament model as modified to take into account slow axoplasmic transport.