The Affinity of Human Erythrocyte Porphobilinogen Synthase for Zn²⁺ and Pb²⁺

Abstract

Porphobilinogen synthase activity has been measured in human erythrocyte lysates supplemented with metal-ion buffers to control free Zn" and Pb'+ concentrations. The enzyme is activated by Zn2+ with a K,,, of 1.6 pM and inhibited by Pb" with a K, of 0.07 pM. Pb" and Zn" appear to compete for a single metal-binding site. The half-time for loss of Zn'+ from the active site, or replacement of Pb'+ by Zn' ', were in the 10-20-min range at 37°C. Zn'~' did not affect the affinity for the substrate 5-aminolevulinate, but Pb" reduced it non-competitively.All the experiments were conducted with a blood sample of the common 1-1 phenotype [Astrin, K.

An Artificial Gene for Human Porphobilinogen Synthase Allows Comparison of an Allelic Variation Implicated in Susceptibility to Lead Poisoning

“…Precise analytical studies using controlled metal ion buffers have shown that the K d for Pb(II) is ϳ20-fold tighter than the K d for Zn(II) (38). As a first step in determining the relative ability of Pb(II) to displace Zn(II) from the two isozymes of human PBGS, equilibrium dialysis studies were carried out at pH 7 at initial Zn(II) concentrations of 1 and 10 M and Pb(II) concentrations of 0 and 20 M. These experiments started with apoenzyme prepared by low pH dialysis.…”

Fundamental Coordination Chemistry, Environmental Chemistry, and Biochemistry of Lead(II)

“…One of the best-documented targets for lead is the zinc enzyme ALAD (or porphobilinogen synthase, PBS) (243,484), which has been used extensively as a biomarker for lead poisoning (see Fig. 30 and Section VI.E) (244,(485)(486)(487). d-aminolevulinic acid dehydratase catalyzes the second reaction in the heme-biosynthetic pathway and has been shown to be inhibited by femtomolar concentrations of lead in vitro (K i ¼ 0.07 pM, vs K Zn M ¼ 1.6 pM) (487).…”

Environmental chemical exposures and disturbances of heme synthesis.

“…Lead causes accumulation of zinc protoporphyrin (ZPP) in erythrocytes and large increases of ALA and coproporphyrin in urine. Lead reversibly inhibits ALA-D and also appears to interfere with the function of coproporphyrinogen oxidase and ferrochelatase, possibly by mechanisms other than direct enzyme inhibition (11,98,136,137). It is conceivable that the coproporphyrinuria occurs by the same mechanism as in ALA loading of normal subjects (Biochemistry of Porphyrias).…”