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The aim of this study was to determine the effects of seasonal temperature variation on the functional properties of lactate dehydrogenase (LDH) from white muscle and liver of Norwegian coastal cod (Gadus morhua) and the possible relevance of LDH allelic variability for thermal acclimation. Two groups of fishes were acclimated to 4°C or 12°C for one year. Polymorphism was observed in only one (Ldh-B) of the three Ldh loci expressed in cod liver and/or muscle. Isozyme expression remained unchanged regardless of acclimation temperature (T[sub A]). The products of locus LdhB comprise only 14-19% (depending on the tissue) of total LDH activities and, consequently, differences between phenotypes are negligible in terms of their effect on LDH total performance. No kinetic (K[sup PYR, sub m], K[sup PYR, sub si], V[sub max]) or thermodynamic (E[sub a], ΔG) differences were found among Ldh-B phenotypes. Clear kinetic differences were observed between LDH isoforms in the two tissues. However, the Arrhenius activation energy (E[sub a]) for pyruvate reduction was the same for both tissues (E[sub a]=47 kJ mol[sup -1]) at T[sub A]=12°C. Factors T[sub A], tissue and phenotype did not reveal a significant effect on the Gibbs free energy change (ΔG) of the reaction (55.5 kJ mol[sup -1]). However, at T[sub A]=4°C, the E[sub a] was increased (E[sub a]=53-56 kJ mol[sup -1]) and the temperature dependence of the constant of substrate inhibition for pyruvate (K[sup PYR, sub si]) decreased in both muscle and liver. In conclusion, the strategies of LDH adjustment to seasonal temperature variations in cod involve changes in LDH concentration (quantitative), adjustment of thermodynamic (E[sub a]) and kinetic (K[sup PYR, sub si]) properties of the LDH (modulative) but not the expression of alternative isoforms (qualitative). We assume that the observed increase in E[sub a] and the decrease of temperature dependence of K[sup PYR, sub si] at low T[sub A] is the result of structural changes of the LDH molecule (temperature-driven protein folding). We propose a new mechanism of metabolic compensation of seasonal temperature variations — cold acclimation results in changes in the kinetic and thermodynamic properties of LDH in a way that favours aerobic metabolism through reduction of the competition of LDH for pyruvate in normoxic conditions. [ABSTRACT FROM AUTHOR]