Abstract
Some mammals hibernate in response to harsh environments. Although hibernating mammals may metabolize proteins, the nitrogen metabolic pathways commonly activated during hibernation are not fully characterized. In contrast to the hypothesis of amino acid preservation, we found evidence of amino acid metabolism as three of five key enzymes, including phenylalanine hydroxylase (PAH), homogentisate 1,2-dioxygenase (HGD), fumarylacetoacetase (FAH), involved in phenylalanine and tyrosine catabolism were co-upregulated during hibernation in two distantly related species of bats, Myotis ricketti and Rhinolophus ferrumequinum. In addition, the levels of phenylalanine in the livers of these bats were significantly decreased during hibernation. Because phenylalanine and tyrosine are both glucogenic and ketogenic, these results indicate the role of this catabolic pathway in energy supply. Since any deficiency in the catabolism of these two amino acids can cause accumulations of toxic metabolites, these results also suggest the detoxification role of these enzymes during hibernation. A higher selective constraint on PAH, HPD, and HGD in hibernators than in non-hibernators was observed, and hibernators had more conserved amino acid residues in each of these enzymes than non-hibernators. These conserved amino acid residues are mostly located in positions critical for the structure and activity of the enzymes. Taken together, results of this work provide novel insights in nitrogen metabolism and removal of harmful metabolites during bat hibernation.
Highlights
Hibernation is a controlled reversible reduction of body temperature, metabolic rate, and many other physiological processes of some animals in order to survive in winter [1,2,3,4]
Liver Proteome of Myotis ricketti Expression profiles of liver proteins in hibernating and non-hibernating states of M. ricketti were compared by proteomic analyses (Figures 1A and S1), and the expression of a number of proteins was found to be increased in torpid bats (Table 1)
Since acetoacetate is a catabolite of phenylalanine, these results suggest that the catabolism of phenylalanine is more active in R. ferrumequinum bats during hibernation
Summary
Hibernation is a controlled reversible reduction of body temperature, metabolic rate, and many other physiological processes of some animals in order to survive in winter [1,2,3,4]. Several mechanisms that may prevent muscle atrophy or loss of liver proteins have been proposed, including the production of proteolytic inhibitors [5,6], continual protein synthesis [12,13], maintenance of proper protein folding [11], and preservation of essential amino acids such as phenylalanine (Phe) and tyrosine (Tyr) [12,13,14]. These two amino acids are both glucogenic and ketogenic in energy production. They are precursors of thyroid hormones and catecholamines (e.g., adrenaline, noradrenaline, and dopamine) that can act on adipose tissues to modulate thermal and energy balance during hibernation [15,16]
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