High levels of lead can combine with SH groups in enzymes

High levels of lead can combine with SH groups in enzymes like Ferrochelatase and ALA dehydratase which affects heme metabolism, and subsequently, porphyrin metabolism.

Elevated levels of porphyrin are found in red blood cells and elevated levels of ALA and Coproporphyrin are found in urine, so the rate of synthesis of ALA synthase increases in the absence of heme and gets diminished in its presence due to the heme acting through an aporepressor molecule and hence being a negative regulator for ALA synthase synthesis.