Importance of storage conditions for the stability of zinc- and cadmium-induced metallothionein

We examined the storage stability of metallothionein (MT), a cysteine-rich protein that has diagnostic potential as a cancer marker and in the assessm ent of Zn status and heavy-metal toxicity. MT was rapidly degraded in sampl es of rat whole liver at -20 degreesC or -70 degreesC. MT in supernatants f rom heat-treated rat liver homogenates stored as 1 : 5 dilutions of liver f rom Zn- or Cd-induced rats were stable (recovery >98%) for 100 d at tempera tures of -70 degreesC and -196 degreesC but not at -20 degreesC, regardless of the presence of dithiothreitol (DTT) or argon. The variability of MT me asurement by the Cd-109-hemoglobin affinity assay was however greatest in s amples from Zn-induced rats stored without DTT. The integrity of the MT pro tein in supernatants of heat-treated homogenates stored for 100 d was demon strated by Sephadex G-75 chromatography. When heat-treated supernatants wer e stored as dilute solutions (1 : 125 of liver), MT was unstable regardless of treatment or storage temperature. Our findings show that liver MT is st able for at least 4 mo as a supernatant of a heat-treated homogenate (1 : 5 dilution of liver) when stored at or below -70 degreesC and in the presenc e of DTT.