Metallothionein (MT) are low-molecular weight, thiol-rich, metal-binding proteins, presumably involved in the homeostasis of essential oligoelements, detoxification of toxic heavy metals and scavenging of free radicals. The present review describes the results of our studies on metallothionein in tissues and cells of marine Antarctic organisms. In the eggs of the Antarctic sea urchin Sterechinus neumayeri, metallothionein amount was found to be two order of magnitude below that of closely related temperate species. The full sequences of the metallothioneins of the two temperate species was obtained by Edman degradation of peptide fragments or inferred by the nucleotide sequence of metallothionein cDNAs prepared by reverse transcriptase-polymerase chain reaction (RT-PCR). Despite the lack of detectable metallothionein protein in the antarctic S. neumayeri, an appreciable quantity of metallothionein gene could be detected in the eggs of this species. Comparable observation were made among Antarctic fish, where appreciable amounts of hepatic metallothionein were found in red-blooded species, whilst very little metallothionein, if any, was detected in icefish. Electrophoresis of PCR products showed the presence of a metallothionein cDNA both in red-blooded and haemoglobinless fish, and high levels of metallothionein mRNA were revealed in both fishes by Northern blot analysis of total RNA. The discrepancy between metallothionein and metallothionein mRNA contents observed in both sea urchin and icefish may be the result of particular metabolic conditions determined by the low temperature of the Antarctic environment. In the eggs of Antarctic sea urchins, characterised by a slow development, a large store of maternal metallothionein, typical of temperate species, is not required because metallothionein mRNA is sufficient to fulfil the reduced requirement of zinc from the developing embryo. In icefish, the low metabolic activity together with the lack of adequate oxygenation may lead to a decreased formation of superoxide radicals, thus reducing the needs of a large amount of metallothionein to be used as free radical scavenger.
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