Regional developmental and fractional studies on myelin and other carbonic anhydrases in rat CNS

Myelin carbonic anhydrase (CA) was studied with respect to its development in various brain regions and light and heavy myelin (LM and HM). The data indicate that the specific activity of myelin CA has a clear neuraxial distribution, increasing rostrally. The absolute activities and relative distribution are invariant with age; this suggests the CA activity in myelin is independent of stage and degree of myelination. The studies on HM and LM illustrate that HM, like total myelin, has a constant CA activity during development. In contrast, LM although equal to HM at 14 days, progressively decays to an adult level which is one-fourth that of HM. The distribution of CA in myelin was further investigated by comparing the activity in myelin with that present in the SN4 fraction. The activity in this latter fraction, which is derived from heavy myelin, was found to be 2.2 times higher than that in the myelin fraction. Thus, in the adult there exists an almost ten-fold range of activities among the various myelin fractions, SN4 greater than HM greater than LM. This may indicate a segregation of activity towards the outer lamellae. This segregation may have physiological importance in that it is this region of the sheath which should be integrally involved in control of myelin edema. Evidence indicates that there is an interaction of chloride with the enzyme, and maybe the primary ion moved by CA in order to initiate an osmotic flux out of the sheath. The interaction of chloride with the enzyme is dependent on the CA complex with the membrane in that solubilization and partial (60-fold) purification results in a preparation which is refractory to anions.