Effect of zinc deficiency on histidine metabolism in rats

Rat liver folate metabolism can provide an independent functioning of associated metabolic pathways

Folate metabolism in mammalian cells is essential for multiple vital processes, including purine and pyrimidine synthesis, histidine catabolism, methionine recycling, and utilization of formic acid. It remains unknown, however, whether these processes affect each other via folate metabolism or can function independently based on cellular needs. We addressed this question using a quantitative mathematical model of folate metabolism in rat liver cytoplasm. Variation in the rates of metabolic processes associated with folate metabolism (i.e., purine and pyrimidine synthesis, histidine catabolism, and influxes of formate and methionine) in the model revealed that folate metabolism is organized in a striking manner that enables activation or inhibition of each individual process independently of the metabolic fluxes in others. In mechanistic terms, this independence is based on the high activities of a group of enzymes involved in folate metabolism, which efficiently maintain close-to-equilibrium ratios between substrates and products of enzymatic reactions.

Zinc in Keratinocytes and Langerhans Cells: Relevance to the Epidermal Homeostasis

In the skin, the epidermis is continuously exposed to various kinds of external substances and stimuli. Therefore, epidermal barriers are crucial for providing protection, safeguarding health, and regulating water balance by maintaining skin homeostasis. Disruption of the epidermal barrier allows external substances and stimuli to invade or stimulate the epidermal cells, leading to the elicitation of skin inflammation. The major components of the epidermal barrier are the stratum corneum (SC) and tight junctions (TJs). The presence of zinc in the epidermis promotes epidermal homeostasis; hence, this study reviewed the role of zinc in the formation and function of the SC and TJs. Langerhans cells (LCs) are one of the antigen-presenting cells found in the epidermis. They form TJs with adjacent keratinocytes (KCs), capture external antigens, and induce antigen-specific immune reactions. Thus, the function of zinc in LCs was examined in this review. We also summarized the general knowledge of zinc and zinc transporters in the epidermis with updated findings.

Targeting the gut barrier for the treatment of alcoholic liver disease

Alcohol consumption remains one of the predominant causes of liver disease and liver-related death worldwide. Intriguingly, dysregulation of the gut barrier is a key factor promoting the pathogenesis of alcoholic liver disease (ALD). A functional gut barrier, which consists of a mucus layer, an intact epithelial monolayer and mucosal immune cells, supports nutrient absorption and prevents bacterial penetration. Compromised gut barrier function is associated with the progression of ALD. Indeed, alcohol consumption disrupts the gut barrier, increases gut permeability, and induces bacterial translocation both in ALD patients and in experimental models with ALD. Moreover, alcohol consumption also causes enteric dysbiosis with both numerical and proportional perturbations. Here, we review and discuss mechanisms of alcohol-induced gut barrier dysfunction to better understand the contribution of the gut-liver axis to the pathogenesis of ALD. Unfortunately, there is no effectual Food and Drug Administration-approved treatment for any stage of ALD. Therefore, we conclude with a discussion of potential strategies aimed at restoring the gut barrier in ALD. The principle behind antibiotics, prebiotics, probiotics and fecal microbiota transplants is to restore microbial symbiosis and subsequently gut barrier function. Nutrient-based treatments, such as dietary supplementation with zinc, niacin or fatty acids, have been shown to regulate tight junction expression, reduce intestinal inflammation, and prevent endotoxemia as well as liver injury caused by alcohol in experimental settings. Interestingly, saturated fatty acids may also directly control the gut microbiome. In summary, clinical and experimental studies highlight the significance and efficacy of the gut barrier in treating ALD.

Increased methionine synthetase activity in zinc-deficient rat liver

To study the effect of zinc deficiency on folate metabolism, three groups of male Sprague-Dawley rats (zinc deficient (ZD), restricted-fed (RF + Zn), and ad libitum-fed control (control] were given a semipurified 25% egg white protein diet. The ZD group received less than 10.3 nmol zinc/g of diet, while the RF + Zn and control groups were given 1620 nmol zinc/g of diet. After 6-7 weeks of feeding, severe zinc deficiency developed in ZD rats. Hepatic methionine synthetase activity was increased in the ZD group compared to both the RF + Zn and control groups, but hepatic 5,10-CH2-H4folate reductase activity was similar in all groups. This increased methionine synthetase activity found in zinc-deficient rats might induce secondary alterations in folate metabolism. These changes include significantly lowered plasma folate levels, decreased 5-CH3-H4folate in liver, and increased rates of histidine and formate oxidation. The latter two findings suggest that the available non-5-CH3-H4folate is increased in zinc deficiency.

The effects of severe zinc deficiency on protein turnover in muscle and thymus

Measurements have been made of protein turnover, RNA and DNA in thymus and skeletal muscle from rats fed on a zinc-deficient diet (ZD) for 10 and 17 d, in pair-fed controls (CI) and in muscle from rats fed on the ZD diet for 24 d and then fed on restricted amounts of the deficient diet with (RIZS) or without (RIZD) Zn supplementation, for 8 d. In thymus the ZD diet induced a loss of DNA and protein which was not observed with the CI rats. Accumulation of RNA was less affected but protein synthesis was reduced. In muscle the accumulation of DNA and protein was slowed by the ZD diet, particularly in glycolytic muscles compared with oxidative muscles, and Zn supplementation increased DNA and protein. Protein synthesis and RNA concentrations were reduced in the ZD rats compared with the CI rats, but Zn supplementation at constant restricted food intake did not increase protein synthesis. Muscle protein synthesis per unit RNA varied markedly in the ZD rats after 10 d when the characteristic cycling of the food intakes and body-weight was most pronounced, the highest values being observed in the anabolic phase of the cycle although these were less than values for well-fed controls. The variability was inversely correlated with the plasma Zn levels. The extent of the variability was much less after 17 d and was not apparent in the food-restricted ZD animals. Protein degradation in muscle, assessed as the difference between overall and net protein synthesis, was faster in the ZD rats compared with the CI rats and fluctuated considerably, partly accounting for the cyclic changes in muscle after 10 d, and was entirely responsible after 17 d. The concentration of muscle-free 3-methylhistidine and its urinary excretion rate indicated inconsistent results which could not be satisfactorily interpreted. Plasma insulin was reduced in the ZD rats compared with the CI rats and was insensitive to food intake in contrast to urinary corticosterone excretion which was inversely correlated with the cyclic changes in body-weight and food intake. Furthermore, adrenalectomized rats exhibited increased mortality and reduced cycling of body-weight and food intake. Thus Zn deficiency impairs growth by a combination of reduced food intake, a reduced anabolic response to food due to a reduced capacity for protein synthesis and reduced activation of protein synthesis, possibly reflecting impaired insulin secretion, and an increased catabolic response to the reduced intake in which corticosterone may play a role.