GUANYLYL CYCLASE ACTIVITY IN RAT-BRAIN MYELIN AND WHITE-MATTER

Purified myelin from rat brainstem was found to have an appreciable le vel of guanylyl cyclase activity, as seen in the formation of 3',5'-cy clic GMP from [H-3]GTP at a rate approximately 45% that of whole brain stem. Freshly isolated myelin from pooled rat brainstems was incubated with GTP in an appropriate mixture. This gave rise to 29.9 +/- 3.6 pm ol of 3',5'-cyclic GMP/mg of protein/min measured by HPLC and a simila r result (26.7 +/- 2.6 pmol/mg/min) with I-125-3',5'-cyclic GMP radioi mmunoassay. The latter method applied to the reaction product from who le brainstem gave a value of 56.6 +/- 3.4 pmol/mg/min. In analyzing br ainstem products by HPLC we observed in most trials concurrent formati on of a second radiolabeled product that comigrated with 2',3'-cyclic GMP but that, on further examination, proved not to be that product. I ts identity remains unknown.