Blood Uric Acid Level and IQ: A Study in Twin Families

Uric acid in health and disease: From physiological functions to pathogenic mechanisms

Owing to renal reabsorption and the loss of uricase activity, uric acid (UA) is strictly maintained at a higher physiological level in humans than in other mammals, which provides a survival advantage during evolution but increases susceptibility to certain diseases such as gout. Although monosodium urate (MSU) crystal precipitation has been detected in different tissues of patients as a trigger for disease, the pathological role of soluble UA remains controversial due to the lack of causality in the clinical setting. Abnormal elevation or reduction of UA levels has been linked to some of pathological status, also known as U-shaped association, implying that the physiological levels of UA regulated by multiple enzymes and transporters are crucial for the maintenance of health. In addition, the protective potential of UA has also been proposed in aging and some diseases. Therefore, the role of UA as a double-edged sword in humans is determined by its physiological or non-physiological levels. In this review, we summarize biosynthesis, membrane transport, and physiological functions of UA. Then, we discuss the pathological involvement of hyperuricemia and hypouricemia as well as the underlying mechanisms by which UA at abnormal levels regulates the onset and progression of diseases. Finally, pharmacological strategies for urate-lowering therapy (ULT) are introduced, and current challenges in UA study and future perspectives are also described.

The Role of Uric Acid in Human Health: Insights from the Uricase Gene

Uric acid is the final product of purine metabolism and is converted to allantoin in most mammals via the uricase enzyme. The accumulation of loss of function mutations in the uricase gene rendered hominoids (apes and humans) to have higher urate concentrations compared to other mammals. The loss of human uricase activity may have allowed humans to survive environmental stressors, evolution bottlenecks, and life-threatening pathogens. While high urate levels may contribute to developing gout and cardiometabolic disorders such as hypertension and insulin resistance, low urate levels may increase the risk for neurodegenerative diseases. The double-edged sword effect of uric acid has resurrected a growing interest in urate's antioxidant role and the uricase enzyme's role in modulating the risk of obesity. Characterizing both the effect of uric acid levels and the uricase enzyme in different animal models may provide new insights into the potential therapeutic benefits of uric acid and novel uricase-based therapy.

Emerging Role of Purine Metabolizing Enzymes in Brain Function and Tumors

The growing evidence of the involvement of purine compounds in signaling, of nucleotide imbalance in tumorigenesis, the discovery of purinosome and its regulation, cast new light on purine metabolism, indicating that well known biochemical pathways may still surprise. Adenosine deaminase is important not only to preserve functionality of immune system but also to ensure a correct development and function of central nervous system, probably because its activity regulates the extracellular concentration of adenosine and therefore its function in brain. A lot of work has been done on extracellular 5′-nucleotidase and its involvement in the purinergic signaling, but also intracellular nucleotidases, which regulate the purine nucleotide homeostasis, play unexpected roles, not only in tumorigenesis but also in brain function. Hypoxanthine guanine phosphoribosyl transferase (HPRT) appears to have a role in the purinosome formation and, therefore, in the regulation of purine synthesis rate during cell cycle with implications in brain development and tumors. The final product of purine catabolism, uric acid, also plays a recently highlighted novel role. In this review, we discuss the molecular mechanisms underlying the pathological manifestations of purine dysmetabolisms, focusing on the newly described/hypothesized roles of cytosolic 5′-nucleotidase II, adenosine kinase, adenosine deaminase, HPRT, and xanthine oxidase.

Hyperuricemia-Related Diseases and Xanthine Oxidoreductase (XOR) Inhibitors: An Overview

Uric acid is the final oxidation product of purine metabolism in humans. Xanthine oxidoreductase (XOR) catalyzes oxidative hydroxylation of hypoxanthine to xanthine to uric acid, accompanying the production of reactive oxygen species (ROS). Uric acid usually forms ions and salts known as urates and acid urates in serum. Clinically, overproduction or under-excretion of uric acid results in the elevated level of serum uric acid (SUA), termed hyperuricemia, which has long been established as the major etiologic factor in gout. Accordingly, urate-lowering drugs such as allopurinol, an XOR-inhibitor, are extensively used for the treatment of gout. In recent years, the prevalence of hyperuricemia has significantly increased and more clinical investigations have confirmed that hyperuricemia is an independent risk factor for cardiovascular disease, hypertension, diabetes, and many other diseases. Urate-lowering therapy may also play a critical role in the management of these diseases. However, current XOR-inhibitor drugs such as allopurinol and febuxostat may have significant adverse effects. Therefore, there has been great effort to develop new XOR-inhibitor drugs with less or no toxicity for the long-term treatment or prevention of these hyperuricemia-related diseases. In this review, we discuss the mechanism of uric acid homeostasis and alterations, updated prevalence, therapeutic outcomes, and molecular pathophysiology of hyperuricemia-related diseases. We also summarize current discoveries in the development of new XOR inhibitors.

The Role of Uric Acid and Methyl Derivatives in the Prevention of Age-Related Neurodegenerative Disorders

High uric acid (UA levels have been correlated with a reduced risk of many neurodegenerative diseases through mechanisms involving chelating Fenton reaction transitional metals, antioxidant quenching of superoxide and hydroxyl free radicals, and as an electron donor that increases antioxidant enzyme activity (e.g. SOD. However, the clinical usefulness of UA is limited by its' low water solubility and propensity to form inflammatory crystals at hyperuricemic levels. This review focuses on the role of UA in neuroprotection, as well as potential strategies aimed at increasing UA levels in the soluble range, and the potential therapeutic use of more water-soluble methyl-UA derivatives from the natural catabolic end-products of dietary caffeine, theophylline, and theobromine.

Uric acid: a danger signal from the RNA world that may have a role in the epidemic of obesity, metabolic syndrome, and cardiorenal disease: evolutionary considerations

All human beings are uricase knockouts; we lost the uricase gene as a result of a mutation that occurred in the mid-Miocene epoch approximately 15 million years ago. The consequence of being a uricase knockout is that we have higher serum uric acid levels that are less regulatable and can be readily influenced by diet. This increases our risk for gout and kidney stones, but there is also increasing evidence that uric acid increases our risk for hypertension, kidney disease, obesity, and diabetes. This raises the question of why this mutation occurred. In this article we review current hypotheses. We suggest that uric acid is a danger and survival signal carried over from the RNA world. The mutation of uricase that occurred during the food shortage and global cooling that occurred in the Miocene epoch resulted in a survival advantage for early primates, particularly in Europe. Today, the loss of uricase functions as a thrifty gene, increasing our risk for obesity and cardiorenal disease.

Uric acid and evolution

Uric acid (UA) is the end product of purine metabolism in humans due to the loss of uricase activity by various mutations of its gene during the Miocene epoch, which led to humans having higher UA levels than other mammals. Furthermore, 90% of UA filtered by the kidneys is reabsorbed, instead of being excreted. These facts suggest that evolution and physiology have not treated UA as a harmful waste product, but as something beneficial that has to be kept. This has led various researchers to think about the possible evolutionary advantages of the loss of uricase and the subsequent increase in UA levels. It has been argued that due to the powerful antioxidant activity of UA, the evolutionary benefit could be the increased life expectancy of hominids. For other authors, the loss of uricase and the increase in UA could be a mechanism to maintain blood pressure in times of very low salt ingestion. The oldest hypothesis associates the increase in UA with higher intelligence in humans. Finally, UA has protective effects against several neurodegenerative diseases, suggesting it could have interesting actions on neuronal development and function. These hypotheses are discussed from an evolutionary perspective and their clinical significance. UA has some obvious harmful effects, and some, not so well-known, beneficial effects as an antioxidant and neuroprotector.

The planetary biology of ascorbate and uric acid and their relationship with the epidemic of obesity and cardiovascular disease

Humans have relatively low plasma ascorbate levels and high serum uric acid levels compared to most mammals due to the presence of genetic mutations in l-gulonolactone oxidase and uricase, respectively. We review the major hypotheses for why these mutations may have occurred. In particular, we suggest that both mutations may have provided a survival advantage to early primates by helping maintain blood pressure during periods of dietary change and environmental stress. We further propose that these mutations have the inadvertent disadvantage of increasing our risk for hypertension and cardiovascular disease in today's society characterized by Western diet and increasing physical inactivity. Finally, we suggest that a "planetary biology" approach in which genetic changes are analyzed in relation to their biological action and historical context may provide the ideal approach towards understanding the biology of the past, present and future.

Uric acid nephrolithiasis: proton titration of an essential molecule?

PURPOSE OF REVIEW

The majority of uric acid nephrolithiasis in humans occurs in the absence of frank hyperuricosuria and is primarily a disease of excessively low urinary pH. Uric acid is substantially less soluble than urate salts so in low urine pH urate is protonated, thus favoring precipitation even under what is considered physiologic concentrations of total urinary uric acid/urate. This commentary examines the rationales behind the existence of uric acid in urine and body fluids in vertebrate evolution.

RECENT FINDINGS

The purpose of uric acid in arthropod, avian and reptilian species is to enable nitrogen excretion in solid state without loss of water. The re-emergence of uric acid in higher primates as an end product of metabolism is intriguing since urea functions perfectly well as a nitrogenous waste. Uric acid must purvey important physiologic functions in primate biology. Numerous roles of uric acid as an antioxidant, immune signaling molecule, and a defender of circulatory integrity have recently been proposed.

SUMMARY

There is little doubt that uric acid serves multiple important functions in higher primates. It is also conceivable, however, that this important molecule when present in the wrong concentration or context can lead to undesirable phenotypes.

Hormonal and cytokine effects of uric acid

PURPOSE OF REVIEW

Current evidence supports the role of soluble uric acid as a true mediator of injury, exerting its effects through the induction of growth factors, cytokines, hormones and autacoids. In the present review, we summarize recent studies on the mechanisms involved in the uric acid deleterious effects.

RECENT FINDINGS

Although uric acid is considered an antioxidant in plasma, recent clinical and epidemiological studies have found that hyperuricemia is associated with mortality and development of hypertension, cardiovascular and chronic renal diseases. Experimental studies suggest that uric acid induce its detrimental effects at the cellular level entering to vascular smooth muscle cells (VSMC) via an organic anion transport system, and followed by the activation of specific MAP kinases, nuclear transcription factors, with stimulation of COX-2, PDGF A and C chain, PDGF alpha receptor, and various inflammatory mediators, including C-reactive protein and monocyte chemoattractant protein-1. Physiologically, these effects translate into a rise of arterial pressure, VSMC hypertrophy, tubulointerstitial infiltration and glomerular hypertension in the setting of renal vasoconstriction. Uric acid also promotes endothelial dysfunction through inactivation of NO and arresting the proliferation of endothelial cells. Thus, arteriosclerosis induced by hyperuricemia may be a novel mechanism for the development of essential hypertension.

SUMMARY

Soluble uric acid has important biologic roles. While it acts as an antioxidant, there is also evidence that uric acid has pro-inflammatory and proliferative effects on VSMC, and causes dysfunction of endothelial cells. These cellular mechanisms may translate into why uric acid is associated with renal and cardiovascular disease.

Relationship between blood uric acid level and personality traits

The present study deals with the relationship between blood uric acid level and human behavior. Subjects were 37 MZ and 7 DZ twins aged from 18 to 45 years. In males, blood uric acid level increased with age, while it decreased with age in females. Blood uric acid level was corrected and standardized using regression lines separately for males and females. The distribution of standardized uric acid level corresponded well with the theoretical curve of normal distribution. The intraclass correlation coefficient for standardized uric acid level was r = 0.370 (P less than 0.05) for the 37 MZ twins, but not significant for the 7 DZ twins. These findings suggest that blood uric acid level is genetically controlled. By the analysis of 12 personality traits in YG (Yatabe-Guilford) character test, it was revealed that "General activity" was more controlled by genetically than environmentally. In the evaluation of the correlation between standardized uric acid level and the YG 12 personality traits, significant correlation was observed in "Lack of agreeableness" and "Rhathymia". Since these two personality traits include the factor of "activity", it is concluded that the plasma uric acid level and activity in a broader sense are under genetic control. This conclusion is consistent with the generally accepted view that persons with high uric acid level are more active and energetic than those with low level.