ATP-dependent [3H]Met-enkephalin uptake by bovine adrenal chromaffin granule membrane

Uptake of [3H]Met-enkephalin by purified chromaffin granule membrane isolated from bovine adrenal medulla was investigated. Addition of a single divalent cation, such as Mg2+ or Ca2+, did not affect the uptake. The presence of 3 mM ATP in the incubation medium stimulated the uptake to two times that of the control. The same stimulation effect was also found in the presence of ATP plus a single divalent cation. The ATP-dependent uptake reached its half-maximal level within 5 min after initiation of the reaction at 25 degrees C, and reached a plateau within 10 min. The apparent Km for [3H]Met-enkephalin uptake by the chromaffin granule membrane was about 2.3 X 10(-6) M. GTP, CTP, UTP and ADP did not stimulate enkephalin uptake. Several calcium inhibitors such as trifluoperazine, verapamil and 3,4,5-trimethoxybenzoic acid 8-(N,N-diethylamino)octylester (TMB-8) markedly inhibited enkephalin uptake. The inhibition exceeded 80%, although these chemicals inhibited Ca2+-ATPase activity in chromaffin granule membrane only 30% under the same conditions. Ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetraacetic acid (EGTA), at 3 mM also inhibited uptake about 30%. The results indicate that uptake of Met-enkephalin by chromaffin granule membrane ghosts was driven not only by ATPase activity, but also by some other Ca2+-ATP-mediated mechanism(s).