Catalog Home Page

Detoxification of iron and oxygen radicals by the southern hemisphere lamprey Geotria australis Gray

Harris, Leon Robert (1995) Detoxification of iron and oxygen radicals by the southern hemisphere lamprey Geotria australis Gray. PhD thesis, Murdoch University.

[img]
PDF - Whole Thesis
Available Upon Request

Abstract

The mechanisms whereby the Southern Hemisphere lamprey Geotria australis may minimise the potential toxicity of the extremely high tissue concentration of iron have been examined. Attention has focussed on elucidating mechanisms that prevent iron from interacting with oxygen free radicals and on those characteristics of lamprey ferritin which may allow its iron to be retained very efficiently.

The concentration of total non-haem iron and its ferritin iron component, and the activity of the enzyme superoxide dismutase (SOD), have been measured in the livers of ammocoetes, metamorphosing and young adult (downstream migrating) G. australis. Total non-haem and ferritin iron concentrations in the liver rose significantly (p < 0.05) in the middle and later stages of metamorphosis. Hepatic SOD activity fell sharply immediately after the onset of metamorphosis, before rising significantly to high levels in downstream migrants. The sharp fall in SOD activity at the beginning of metamorphosis is assumed to be related to the marked decline in plasma iron which occurs at the onset of this non-trophic phase in the life cycle.

The relationship between SOD activity and iron levels was further examined in the livers of adult upstream migrating G. australis that were naturally and artificially loaded with iron. A rise in iron levels over the 12 months of the upstream migration was tracked by the activity of SOD. Weekly intramuscular injections of iron as horse spleen ferritin into adult G. australis over ten weeks resulted in a progressive increase in that form of iron in the serum. However, as with control animals, the liver ferritin of injected lampreys consisted of one subunit type, whose Mr differed from horse spleen ferritin. The lampreys had thus converted horse spleen ferritin iron into endogenous ferritin iron, presumably in their liver. Marked rises in hepatic non-haem iron during the first two weeks and between weeks 8 and 10 of iron injections were accompanied by pronounced increases in SOD activity. A levelling off of iron concentration between weeks 2 and 8 was accompanied by a decline in SOD activity, even though non-haem iron levels were well above those of control animals. Enhanced SOD activity may therefore only be required when there is an elevated flux of iron in the liver through low molecular weight intermediates.

Ceruloplasmin activity in larval and adult lamprey plasma was negligible compared to that of human plasma, despite the fact that the copper content of plasma in lampreys and humans was comparable. This major antioxidant system found in higher vertebrates is thus apparently absent from lamprey plasma.

Adult lamprey liver ferritin is composed of two principal isoforms with isoelectric points of approximately 5.5, and appears to consist of a single type of subunit with a molecular mass of 20 300 Da, intermediate between the molecular mass of the heavy and light chains of higher vertebrates. The iron core contained within adult liver ferritin was predominantly (90%) crystalline, with about 10% in a non-crystalline phase and had a mean width of 7.0 nm. The ability of lamprey ferritin to resist the mobilisation of iron from its core by a superoxide-generating system was also examined and compared with those of other vertebrates. In contrast to ferritins from human, rat, and trout liver those of lamprey liver and horse spleen did not release iron in a predominantly superoxide-dependent manner.

The ability of G. australis to respond to elevated iron levels by enhancing the activity of the protective enzyme SOD, together with the resistance of lamprey ferritin to superoxide-mediated and superoxide-independent iron release, probably accounts for the tolerance of this species to iron levels that are well in excess of those considered to be toxic in humans and from which lampreys appear to suffer no harm.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: repository@murdoch.edu.au. Thank you.
Supervisor(s): Macey, David, Cake, Max and Potter, Ian
URI: http://researchrepository.murdoch.edu.au/id/eprint/52021
Item Control Page Item Control Page