Although the binding component measured in our assays was not definitively identified as MT, it is heat stable, comigrates with rabbit liver MT II on SDS-PAGE gels (< 10 kDa), and gives the same anomalous staining with Coomassie blue (indicating a low content of aromatic amino acids; unpublished results). This protein also resembles MT in its Cd inducibility. Indeed, a single CdCl2 injection up-regulates MT-like activity to the same extent in testis as in kidney and liver. This is surprising because MT protein and mRNA in rodent testis are refractory to Cd induction. Although the distribution of MT-like activity in Squalus testis is stage related (PrM = PoM > GZ = M), the pattern differs from that predicted by tissue Cd levels. proventil inhaler
Moreover, increased levels of MT-like activity after CdCl2 pretreatment do not affect the 109Cd-accumulating gradient, nor is the fold increase (4-fold) sufficient to account for the increase in testicular Cd (> 160fold). Although we cannot rule out the possibility that the standard Cd hemoglobin assay underestimates the number of Cd-binding sites (due to occupancy by radioinert Cd), the data are consistent with the view that MT is not the primary determinant of stage-related Cd accumulation.