Structure and time-dependent behavior of acetylated and non-acetylated forms of a molluscan metallothionein.

Cadmium-induced metallothionein in a mollusc, the oyster Crassostrea virginica, occurs in both blocked and unblocked forms (Roesijadi, G., Kielland, S.L. and Klerks, P. (1989) Arch. Biochem. Biophys. 273, 403-413). The block, which is the sole difference in the structure of the two proteins, was identified as an acetyl group with use of tandem mass spectrometry. The blocked and unblocked proteins carried N-acetylserine and serine, respectively, at the N-terminus and were designated CvNAcMT and CvMT. Only CvNAcMT was detected under basal conditions. Both forms were induced by Cd. Pulse-labeling with [35S]cyteine at specified times during exposure showed that the rate of CvNAcMT synthesis in gills increased rapidly, initially exceeding that of CvMT, then declined to the rate attained by CvMT. Turnover rates for Cd-induced CvMT and CvNAcMT were similar to each other. They appeared to be faster when measured in the absence of Cd in the external medium (k = 0.18 and 0.16/day, respectively), than in its presence (k = 0.03 and 0.06/day, respectively).