INDUCTION BY ZINC OF SPECIFIC METALLOTHIONEIN ISOFORMS IN HUMAN MONOCYTES

A low-molecular-mass zinc-containing protein was isolated by gel perme ation 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 identifi ed the two major charge-separable fractions and an occasionally occurr ing third fraction as metallothionein-1, metallothionein-2 and metallo thionein-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 b y screening a human monocyte cDNA library. We can find no previous obs ervation in the literature of metallothionein-1g at both the protein a nd RNA level in a non-tumour cell, and of metallothionein-0 in a non-f etal cell or tissue. Since isoform-specific polymerase-chain-reaction amplification showed the presence of metallothionein-0 mRNA in zinc-in duced but not in untreated monocytes, these cells can be used as an in vitro system to investigate the expression of this previously conside red fetal isoform.