Investigations on myelinogenesis in vitro: Regulation of 5′‐nucleotidase activity by thyroid hormone in cultures of dissociated cells from embryonic mouse brain

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Abstract
The developmental pattern of the myelin‐associated 5′‐nucleotidase and its regulation by L‐3,3′,5,‐triiodothyronine (T3) have been demonstrated in a culture system of cells dissociated from embryonic mouse brain. Hypothyroid calf serum containing low levels of T3 (31 ng/100 ml), and thyroxine, T4 (< 1 μg/ml), was used in the culture medium in place of normal calf serum (T3, 103 ng/100ml; T4, 5.7 μg/ml) to render the cultures responsive to exogenously added T3. By means of T3 supplementation, the lower levels of enzyme activity observed in the cultures grown in the presence of hypothyroid calf‐serum containing medium could be restored to a considerable extent although not completely to normal values. Halfmaximal stimulatory effect was obtained at 3.9 × 10−8 M T3 concentration. Among the various substrates tested, 5′‐AMP, 5′‐UMP and 5′‐CMP were equally good, while 5′‐GMP yielded approximately half the activity.