The effect of lead in vitro on the pH-activity relationship of human erythrocytic delta-aminolaevulinic acid dehydratase (delta-ALAD) and on the assessment of lead exposure with ratios of delta-ALAD activity measured at specific pH values was investigated. The addition of lead nitrate to whole blood at concentrations ranging from 0.40 to 8.1 mumole Pb2+/1, for periods of contact ranging from 16 h to 20 days at 4 degrees C, resulted in a time and dose dependent shift of the enzyme's pH optimum to a more acid value. The pH optimum shift obtained at raised lead concentrations or after long periods of contact at 4 degrees C, in both, closely approximated that observed in vivo. The loss of enzyme activity, however, was significantly less in vitro than that in vivo for similar whole blood lead concentrations. These findings indicate that the presence of trace amounts of lead in blood collection devices can seriously affect results obtained with the pH activity ratio method of assessing lead exposure.
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