Bioavailability of lead to juvenile swine dosed with soil from the Smuggler Mountain NPL Site of Aspen, Colorado.

Bioavailability of lead (Pb) has become an issue in quantifying exposure of sensitive populations and, where necessary, establishing cleanup levels for contaminated soil. Immature swine were used as a model for young children to estimate the degree to which Pb from two fully characterized composite samples from the Smuggler Mountain Superfund Site in Aspen, Colorado may be bioavailable to resident children. The composite soils contained 14,200 and 3870 micrograms Pb/g of soil. Relative and absolute enteric bioavailabilities of Pb in soil (oral dose groups of 75,225, and 675 micrograms Pb/kg body wt/day) were estimated by comparison with an orally administered soluble Pb salt (lead acetate = PbAc2.3H2O) (dose groups of 0, 75, and 225 micrograms Pb/kg body wt/day) and an intravenously administered aqueous solution of Pb (100 micrograms Pb/kg/ day) from the same trihydrate salt administered daily for 15 days to 50 juvenile swine. The biological responses (area under the blood Pb concentration-time curve, and the terminal liver-, kidney-, and bone-lead concentrations) produced by Pb from PbAc2.3H2O and lead-contaminated soils were determined. This study revealed Pb from soil containing 14,200 micrograms Pb/g of soil had a bioavailability relative to Pb from PbAc (RBA), ranging from 56% based on the area under the blood lead concentration-time curve (AUC) versus dose, to 86% based on calculations from liver-Pb loading versus dose. Similarly, Pb from soil containing 3870 micrograms Pb/g of soil had an RBA ranging from 58% based on the AUC versus dose, to 74% based on calculations from liver- and kidney-Pb loading versus dose. Bioavailability of Pb in soils may be more or less than EPA's default RBA of 60%, therefore, measuring site-specific RBAs provides a basis for improved exposure and risk assessment.

Antibodies against the 53 kDa glycoprotein inhibit the rotational dynamics of both the 53 kDa glycoprotein and the Ca(2+)-ATPase in the sarcoplasmic reticulum membrane.

The purpose of this study is to better define the relationship of the 53 kDa glycoprotein (GP-53) of the sarcoplasmic reticulum (SR) to other SR proteins. Towards that end the effects of antibodies against GP-53 on the rotational dynamics of maleimide spin-labeled proteins of SR of rabbit skeletal muscle were investigated. The labeling protocol used in this study provided 1.6 +/- 0.3 moles spin label incorporated per 10(5) g SR protein. Labeling specificity studies indicated that nearly 70% of the label bound specifically to the Ca(2+)-ATPase, with the remainder bound to GP-53. Using saturation-transfer electron paramagnetic resonance (ST-EPR), it was determined that the rotational mobility (i.e., the rate of rotation) of the spin-labeled SR proteins decreased greater than 5-fold upon preincubation of MSL-SR with an antiserum against the GP-53, while preincubation of MSL-SR with preimmune serum had no effect. Preincubation of MSL-SR with a monoclonal antibody against the GP-53 produced a 4-fold decrease in the rotational mobility of the MSL-SR proteins compared to control measurements. Further, these effects showed a marked calcium dependence: the decrease in the rotational mobility of the MSL-SR proteins preincubated with anti-GP-53 antibodies in 500 microM Ca2+ was 3-6-fold greater than that of MSL-SR preincubated with antibodies in 5 mM EGTA. While MSL was bound to both Ca(2+)-ATPase and GP-53, model calculations indicated that the decreases observed in the rotational mobility of the MSL-SR proteins caused by the anti-GP-53 monoclonal antibodies were too large to be accounted for by effects on GP-53 alone. The calculations suggest that the rotational rate of Ca(2+)-ATPase was also diminished by anti-GP-53 monoclonal antibodies, indicating an interaction between GP-53 and Ca(2+)-ATPase in the SR membrane.

Influence of the 53 kDa glycoprotein on the cooperativity of the Ca(2+)-ATPase of the sarcoplasmic reticulum.

Previous results from this laboratory suggest that the 53 kDa glycoprotein (GP-53) of rabbit skeletal muscle sarcoplasmic reticulum membrane (SR) may influence coupling between Ca2+ transport and ATP hydrolysis by the Ca(2+)-ATPase. Here we report evidence that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase. The ATPase activity of the Ca(2+)-ATPase displays negative cooperative dependence (Hill coefficient n less than 1) on [MgATP] and has positive cooperative dependence (n greater than 1) on [Ca2+]free. We have determined the degree of cooperativity for native SR vesicles, SR preincubated with antiserum against GP-53 or preimmune serum, and SR partially extracted with KCl-cholate. Our results show that SR preincubated with preimmune serum or SR treated with cholate in 50 mM KCl (yielding membranes rich in GP-53) demonstrate a cooperative dependence of Ca(2+)-ATPase activity on both [ATP] and [Ca2+] similar to that of untreated SR. SR preincubated with anti-GP-53 antiserum (which causes an uncoupling of Ca2+ transport from ATP hydrolysis) or SR extracted with cholate in 1 M KCl (yielding membranes depleted of GP-53) displays decreased positive cooperative dependence on [Ca2+] and decreased negative cooperative dependence on [ATP]. The results are consistent with the interpretation that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase.