Biochemical studies of thiamine deficiency in Baltic salmon (Salmo salar)

Abstract: The first part of this thesis is about the development of a method to inject various substances into the yolk of fish eggs. The method was named nanoinjection and enabled the injection of volumes as small as 10-9 L into the yolk of single fish eggs directly after fertilization, with insignificant mortality due to the injection. The nanoinjection method has been shown to be useful in studies of the toxic effects of environmental contaminants in early life stages and is considered to mimic maternal transfer of anthropogenic and natural substances to the egg. The subsequent parts in this thesis concern thiamine (vitamin B1) and the consequences of thiamine deficiency. Since 1995, it is known that the Baltic salmon (Salmo salar) females transfer insufficient amounts of thiamine into their eggs for their offspring to survive, a condition known as the M74 syndrome. The embryonic and larval development of salmon from the river Dalälven (Sweden) was studied during the seasons of 1994–1995 and 1995–1996. Disorders at hatch and symptoms of thiamine deficiency were described. A parallel study compared early development in cod (Gadus morhua) from the Baltic Sea to cod from the Barents Sea. Both salmon and cod from the Baltic Sea showed a number of disorders at hatch. Nanoinjections of thiamine solutions into newly fertilized eggs from Baltic salmon protected the larvae from the M74 syndrome. Larvae with the M74 syndrome had low hepatic activities of the thiamine diphosphate-dependent enzymes transketolase (TK) and the ?-ketoglutarate dehydrogenase complex (KGDHC). Also, apparently healthy Baltic salmon yolk-sac larvae showed reduced activities of TK and KGDHC, compared to the offspring from intraperitoneal thiamine-injected females. The result suggested a sublethal thiamine deficiency in the salmon population. The hypothesis that redox-cycling substances affect thiamine metabolism, was tested in rainbow trout (Oncorhynchus mykiss) juveniles. Exposure to paraquat caused a ten-fold increase in the activity of glutathione reductase and elevated the activity of glucose-6-phosphate dehydrogenase, the rate limiting enzyme in the pentose-phosphate shunt. Despite the nine week period of oxidative stress, neither the TK activity nor thiamine concentrations were affected. The underlying cause(s) of the low thiamine concentration in the Baltic salmon is still unknown.