Induction by zinc of specific metallothionein isoforms in human monocytes

Abstract

A low‐molecular‐mass zinc‐containing protein was isolated by gel permeation and anion‐exchange chromatography of lysates of human monocytes induced with zinc acetate. Characterization by sodium dodecyl sulphate/polyacrylamide gel electrophoresis and amino acid sequencing identified the two major charge‐separable fractions and an occasionally occurring third fraction as metallothionein‐1, metallothionein‐2 and metallothionein‐0, respectively. Metallothionein‐1 was shown to consist of a mixture of isoforms, confirmed as metallothionein‐1e, metallothionein‐1g and metallothionein‐11 by comparison with cDNA sequences obtained by screening a human monocyte cDNA library. We can find no previous observation in the literature of metallothionein‐1g at both the protein and RNA level in a non‐tumour cell, and of metallothionein‐0 in a non‐fetal cell or tissue. Since isoform‐specific polymerase‐chain‐reaction amplification showed the presence of metallothionein‐0 mRNA in zinc‐induced but not in untreated monocytes, these cells can be used as an in vitro system to investigate the expression of this previously considered fetal isoform.