PPARα-activation results in enhanced carnitine biosynthesis and OCTN2-mediated hepatic carnitine accumulation

In fasted rodents hepatic carnitine concentration increases considerably which is not observed in PPARα−/− mice, indicating that PPARα is involved in carnitine homeostasis. To investigate the mechanisms underlying the PPARα-dependent hepatic carnitine accumulation we measured carnitine biosynthesis enzyme activities, levels of carnitine biosynthesis intermediates, acyl-carnitines and OCTN2 mRNA levels in tissues of untreated, fasted or Wy-14643-treated wild type and PPARα−/− mice. Here we show that both enhancement of carnitine biosynthesis (due to increased γ-butyrobetaine dioxygenase activity), extra-hepatic γ-butyrobetaine synthesis and increased hepatic carnitine import (OCTN2 expression) contributes to the increased hepatic carnitine levels after fasting and that these processes are PPARα-dependent.     

PPAR alpha-activation results in enhanced carnitine biosynthesis and OCTN2-mediated hepatic carnitine accumulation

In fasted rodents hepatic carnitine concentration increases considerably which is not observed in PPAR alpha-/- mice, indicating that PPAR alpha is involved in carnitine homeostasis. To investigate the mechanisms underlying the PPAR alpha-dependent hepatic carnitine accumulation we measured carnitine biosynthesis enzyme activities, levels of carnitine biosynthesis intermediates, acyl-carnitines and OCTN2 mRNA levels in tissues of untreated, fasted or Wy-14643-treated wild type and PPAR alpha-/- mice. Here we show that both enhancement of carnitine biosynthesis (due to increased gamma-butyrobetaine dioxygenase activity), extra-hepatic gamma-butyrobetaine synthesis and increased hepatic carnitine import (OCTN2 expression) contributes to the increased hepatic carnitine levels after fasting and that these processes are PPAR alpha-dependent.     

Sites and regulation of carnitine biosynthesis in mammals

Although the pathway of carnitine biosynthesis in mammals is known, the location of active synthesis of carnitine and regulation of the pathway have not been clearly defined. Studies in several laboratories have shown that the enzymes that collectively convert epsilon-N-trimethyllysine (epsilon-N-TML) to gamma-butyrobetaine are found in all tissues studied in rats and humans, but distribution of the final enzyme of the pathway, gamma-butyrobetaine, 2-oxoglutarate dioxygenase (gamma-butyrobetaine hydroxylase) is variable from one species to another. Evidence from studies in rats and humans indicates that uptake and metabolism of epsilon-N-TML by the kidney is necessary for carnitine biosynthesis from circulating epsilon-N-TML. Limited data now available suggest that some of the intracellularly derived epsilon-N-TML is metabolized to gamma-butyrobetaine and carnitine in the tissue of origin, and some is released into the circulation. epsilon-N-TML in mammals is apparently derived from lysine residues in proteins, which are methylated and later released by protein hydrolysis. This source probably provides sufficient substrate for carnitine biosynthesis. Carnitine biosynthesis from epsilon-N-TML is not regulated by end-product feedback mechanisms. Hepatic gamma-butyrobetaine hydroxylase activity in rats and humans is developmentally regulated, and is increased by dietary L-thyroxine in adult rats. No other mechanisms for regulation of carnitine biosynthesis have been identified.     

6-Phosphofructo-1-kinase isoenzymes of the jejunal mucosa of rabbit, rat and mouse

1. 6-Phosphofructo-1-kinase (PFK) isoenzymes were studied in the jejunal mucosa of rabbit, rat and mouse. 2. The rat mucosal enzyme was found to be very similar to, although not identical with, the mouse mucosal enzyme, as the physical and regulatory properties of these two enzymes were nearly similar except that the immunological studies were dissimilar. 3. PFK prepared from rabbit mucosa showed different and distinct properties from the rat and mouse mucosal PFK when studied by (NH4)2SO4-precipitation, polyacrylamide gel electrophoresis, immunological cross-reactivity and regulatory properties. 4. The difference between the rabbit enzyme and the rat or mouse enzymes is suggested to be due to the lower rate of glycolysis observed in the rabbit jejunal mucosa as the total enzyme activities of the rabbit were found to be less than half of those activities of the rat and mouse mucosa. 5. The dissimilarities among the species in mucosal isoenzymes obtained in the present study are rather expected since the term isoenzyme is now properly reserved for forms that have been shown to be genetically distinct as shown for different tissues in the same species. Such multigenic control does not appear to have been established for the same tissue in different species.     

Alteration of Cerebral Taurine Biosynthesis in Spontaneously Hypertensive Rats

Abstract: Cerebral taurine biosynthesis in a spontaneously hypertensive rat (SHR) has been studied. Cysteine sulfinic acid (CSA) and cysteic acid (CA), possible key intermediates in taurine biosynthesis, were found in the rat brain, whereas no cysteamine-cystamine was detected. In the brain of SHR, a statistically significant decrease in the contents of CSA, CA, and taurine was noted in the cerebellum, hypothalamus, and striatum as compared with normotensive Wistar Kyoto rats. Similarly, it was demonstrated that the activity of cysteine dioxygenase, the enzyme catalyzing cysteine to CSA, was attenuated significantly in the same brain areas of SHR. In contrast, no alteration in the activity of CSA decarboxylase, the enzyme converting CSA to hypotaurine or CA to taurine, was observed. A decline in the percent conversion of [¹⁴C]cysteine to [¹⁴C]taurine was found also in tissue homogenates from the cerebellum, hypothalamus, and striatum of SHR, indicating that the declines in taurine content may be due to an attenuation of taurine biosynthesis, possibly at the step involving cysteine dioxygenase.     

Carnitine: a nutritional, biosynthetic, and functional perspective

Carnitine status in humans is reported to vary according to body composition, gender, and diet. Plasma carnitine concentration positively correlates with the dietary intake of carnitine. The content of carnitine in foodstuff is based on old and inadequate methodology. Nevertheless, dietary carnitine is important. The molecular biology of the enzymes of carnitine biosynthesis has recently been accomplished. Carnitine biosynthesis requires pathways in different tissues and is an efficient system. Overall biosynthesis is determined by the availability of trimethyllysine from tissue proteins. Carnitine deficiency resulting from a defect in biosynthesis has yet to be reported.

The role of carnitine in long-chain fatty acid oxidation is well defined. Recent evidence supports a role for the voltage-dependent anion channel in the transport of acyl-CoAs through the mitochondrial outer membrane. The mitochondrial outer membrane carnitine palmitoyltransferase-I in liver can be phosphorylated and when phosphorylated the sensitivity to malonyl-CoA is greatly decreased. This may explain the change in sensitivity of liver carnitine palmitoyltransferase-I observed during fasting and diabetes. Recently reported data clarify the role of carnitine and the carnitine transport system in the interplay between peroxisomes and mitochondrial fatty acid oxidation. Lastly, the buffering of the acyl-CoA/CoA coupled by carnitine reflects intracellular metabolism. This mass action effect underlies the use of carnitine as a therapeutic agent. In summary, these new observations help to further our understanding of the molecular aspects of carnitine in medicine.     

Carnitine synthesis and uptake into cells are stimulated by fasting in pigs as a model of nonproliferating species

In rodents, fasting increases the carnitine concentration in the liver by an up-regulation of enzymes of hepatic carnitine synthesis and novel organic cation transporter (OCTN) 2, mediated by activation of peroxisome proliferator-activated receptor (PPAR) α. This study was performed to investigate whether such effects occur also in pigs which like humans, as nonproliferating species, have a lower expression of PPARα and are less responsive to treatment with PPARα agonists than rodents. An experiment with 20 pigs was performed, which were either fed a diet ad-libitum or fasted for 24 h. Fasted pigs had higher relative mRNA concentrations of the PPARα target genes carnitine palmitoyltransferase 1 and acyl-CoA oxidase in liver, heart, kidney, and small intestinal mucosa than control pigs, indicative of PPARα activation in these tissues (P<.05). Fasted pigs had a higher activity of γ-butyrobetaine dioxygenase (BBD), enzyme that catalyses the last step of carnitine biosynthesis in liver and kidney, and higher relative mRNA concentrations of OCTN2, the most important carnitine transporter, in liver, kidney, skeletal muscle, and small intestinal mucosa than control pigs (P<.05). Fasted pigs moreover had higher concentrations of free and total carnitine in liver and kidney than control pigs (P<.05). This study shows for the first time that fasting increases the activity of BBD in liver and kidney and up-regulates the expression of OCTN2 in various tissues of pigs, probably mediated by PPARα activation. It is concluded that nonproliferating species are also able to cover their increased demand for carnitine during fasting by an increased carnitine synthesis and uptake into cells.     

Activation and transport of fatty acids in ovarian mitochondria: effect of Lh

Enzymes of fatty acid activation and transport were studied in luteinized rat ovaries. Luteal mitochondria were found to contain high levels of palmitoyl-CoA synthetase and carnitine palmitoyl-transferase activities. In addition, studies on the effect of palmitate concentration on palmitoyl-CoA synthetase activity revealed the possible existence of two forms of the enzyme: Km values of 0.34 mM and 21.33 mM, with Vmax of 3.64 and 66.67 nmoles/min/mg mitochondrial protein respectively, were obtained for the two activities. Similar kinetic data for carnitine palmitoyl-transferase activity in intact mitochondria are a Km of 21 microM and a Vmax of 18.2 nmoles/min/mg mitochondrial protein. Only one activity of this enzyme could be detected in luteal mitochondria. It appears that the activities of both enzymes were not affected by prior administration of LH in vivo. The possibility that this negative finding was due to the experimental procedures employed, rather than a reflection of the situation in vivo, could not be discounted, although its more likely that these two enzymes are probably not locus of LH stimulation. The results indicate that fatty acid oxidation is an important metabolic capability of luteal mitochondria, and support the view regarding the lipid nature of the respiratory fuel of ovarian tissue.     

Exogenous carnitine application augments transport of fatty acids into mitochondria and stimulates mitochondrial respiration in maize seedlings grown under normal and cold conditions

This study aimed to investigate whether exogenous application of carnitine stimulates transportation of fatty acids into mitochondria, which is an important part of fatty acid trafficking in cells, and mitochondrial respiration in the leaves of maize seedlings grown under normal and cold conditions. Cold stress led to significant increases in lipase activity, which is responsible for the breakdown of triacylglycerols, and carnitine acyltransferase (carnitine acyltransferase I and II) activities, which are responsible for the transport of activated long-chain fatty acids into mitochondria. While exogenous application of carnitine has a similar promoting effect with cold stress on lipase activity, it resulted in further increases in the activity of carnitine acyltransferases compared to cold stress. The highest activity levels for these enzymes were recorded in the seedlings treated with cold plus carnitine. In addition, these increases were correlated with positive increases in the contents of free- and long-chain acylcarnitines (decanoyl-l-carnitine, lauroyl-l-carnitine, myristoyl-l-carnitine, and stearoyl-l-carnitine), and with decreases in the total lipid content. The highest values for free- and long-chain acylcarnitines and the lowest value for total lipid content were recorded in the seedlings treated with cold plus carnitine. On the other hand, carnitine with and without cold stress significantly upregulated the expression level of citrate synthase, which is responsible for catalysing the first reaction of the citric acid cycle, and cytochrome oxidase, which is the membrane-bound terminal enzyme in the electron transfer chain, as well as lipase. All these results revealed that on the one hand, carnitine enhanced transport of fatty acids into mitochondria by increasing the activities of lipase and carnitine acyltransferases, and, on the other hand, stimulated mitochondrial respiration in the leaves of maize seedlings grown under normal and cold conditions.     

gamma-Butyrobetaine hydroxylation to carnitine in mammalian kidney.

Activity of γ-butyrobetaine hydroxylase (γ-butyrobetaine, 2-oxoglutarate dioxygenase; EC 1.14.11.1) in liver and kidney of several mammalian species was assayed by measurement of tritium release from γ-[2,3-³H]butyrobetaine. Crude extracts from cat, hamster, rabbit, and Rhesus monkey kidneys effectively converted γ-butyrobetaine to carnitine. In these species, the levels of hydroxylating activity in kidney exceeded or nearly equaled the level of γ-butyrobetaine hydroxylase activity in the corresponding liver. In contrast, dog, guinea pig, mouse, and rat kidney exhibited no or insignificant capacity to hydroxylate γ-butyrobetaine. The notion that the liver is the exclusive or primary site of carnitine synthesis must be reconsidered at least for some mammalian species.     

Submitochondrial localization of 6-N-trimethyllysine dioxygenase - implications for carnitine biosynthesis

The first enzyme of carnitine biosynthesis is the mitochondrial 6-N-trimethyllysine dioxygenase, which converts 6-N-trimethyllysine to 3-hydroxy-6-N-trimethyllysine. Using progressive membrane solubilization with digitonin and protease protection experiments, we show that this enzyme is localized in the mitochondrial matrix. Latency experiments with intact mitochondria showed that 3-hydroxy-6-N-trimethyllysine formation is limited by 6-N-trimethyllysine transport across the mitochondrial inner membrane. Because the subsequent carnitine biosynthesis enzymes are cytosolic, after production, 3-hydroxy-6-N-trimethyllysine must be transported out of the mitochondria by a putative mitochondrial 6-N-trimethyllysine/3-hydroxy-6-N-trimethyllysine transporter system. This transport system represents an additional step in carnitine biosynthesis that could have considerable implications for the regulation of carnitine biosynthesis.     

Activities of serine palmitoyltransferase (3-ketosphinganine synthase) in microsomes from different rat tissues

Serine palmitoyltransferase [EC 2.3.1.50] catalyzes the first unique reaction of sphingolipid biosynthesis. To determine whether or not different rat tissues are capable of initiating this pathway, its activity was determined for microsomes from rat liver, lung, brain, kidney, intestine, spleen, muscle, heart, pancreas, testes, ovary, and stomach. Serine palmitoyltransferase was found in every tissue, and, when compared to the microsomal glycerol 3-phosphate acyltransferase, the activities correlated directly with their sphingomyelin levels as a percentage of total phospholipids. This suggests that the activities were comparable to expected cellular needs for long-chain bases, if the initial enzymes of glycerolipid and sphingolipid biosynthesis influence the phospholipid composition of cells by determining the relative partitioning of fatty acyl-CoA's toward these two lipid classes. Serine palmitoyltransferase activities were also determined using different fatty acyl-CoA's and were consistently greatest with CoA thioesters of saturated fatty acids with 16 +/- 1 carbon atoms. This suggests that the predominance of 18-carbon long-chain bases in vivo is due to the higher activity of this enzyme with palmitoyl-CoA. Together, these findings indicate a role for serine palmitoyltransferase in regulating both the type and amount of long-chain bases found in tissues.     

Inter-tissue and inter-species characteristics of the mitochondrial carnitine palmitoyltransferase enzyme system

Properties of the carnitine palmitoyltransferase (EC 2.3.1.21) (CPT) enzyme system were compared in isolated mitochondria from a range of tissues in rodents, monkey, and man. Common features were as follows: (a) while membrane-bound, CPT I, but not CPT II, was inhibited reversibly by malonyl-coenzyme A (CoA) and irreversibly by CoA esters of certain oxirane carboxylic acids; (b) the detergent, Tween-20, readily solubilized CPT II in active form while leaving CPT I membrane associated and catalytically functional; (c) octyl glucoside and Triton X-100 released active CPT II but caused essentially complete loss of CPT I activity. Use of [3H]tetradecylglycidyl-CoA, a covalent ligand for CPT I, yielded estimates of the enzyme's monomeric molecular size: approximately 86 kDa in non-hepatic tissues and approximately 90-94 kDa in liver, depending upon species. A polyclonal antibody to purified rat liver CPT II recognized a single protein in each tissue; its apparent molecular mass was approximately 70 kDa in all rat tissues and approximately 68 kDa in all mouse tissues as well as monkey and human liver. On Northern blot analysis a rat liver CPT II cDNA probe detected a single approximately 2.5-kilobase mRNA in all rat and mouse tissues examined. The following points are emphasized. First, CPT I and II are different proteins. Second, within a species CPT II, but not CPT I, is probably conserved across tissue lines. Third, slight variations in size of both enzymes were found in different species, although, at least in the case of CPT II, significant amino acid identity exists among the various isoforms. Fourth, CPT I, unlike CPT II, requires membrane integrity for catalytic function. Finally, the strategic use of detergents provides a simple means of discriminating between the two enzyme activities.     

Ubiquinone biosynthesis in rat liver peroxisomes

The possibility that ubiquinone biosynthesis is present in rat liver peroxisomes was investigated. The specific activity of trans-prenyltransferase was 30% that of microsomes, with a pH optimum of around 8. trans-Geranyl pyrophosphate was required as a substrate and maximum activity was achieved with Mn(2+). Several detergents specifically inactivated the peroxisomal enzyme. The peroxisomal transferase is present in the luminal soluble contents, in contrast to the microsomal enzyme which is a membrane component. The treatment of rats with a number of drugs has demonstrated that the activities in the two organelles are subjected to separate regulation. Nonaprenyl-4-hydroxybenzoate transferase has about the same specific activity in peroxisomes as in microsomes and like the transferase activity, its regulation differs from the microsomal enzyme. The results demonstrate that peroxisomes are involved in ubiquinone biosynthesis, and at least two enzymes of the biosynthetic sequence are present in this organelle.     

Mammalian enzymes of trimethyllysine conversion to trimethylaminobutyrate

The biosynthesis of carnitine proceeds from trimethyllysine (TML) by beta-hydroxylation by a liver or kidney mitochondrial enzyme, which requires oxygen, alpha-ketoglutarate, ferrous iron, and ascorbate. This dioxygenase is rapidly inactivated by preincubation with Fe2+, but not Fe3+. The evidence suggests that superoxide anion is involved in the hydroxylation. beta-Hydroxytrimethyllysine undergoes aldol cleavage to glycine and trimethylaminobutyraldehyde under the influence of serine hydroxymethyltransferase and possibly a specific aldolase. The next step, the aldehyde oxidation, is catalyzed by a specific NAD-dependent aldehyde dehydrogenase from liver cytosol. The product, trimethylaminobutyrate, is then hydroxylated by a cytosolic dioxygenase to carnitine. This enzyme, which has the same cofactor requirements as TML hydroxylase, is found in the liver of all species examined, but is absent from the kidney of some species.     

Residue 234 is a master switch of the alternative-substrate activity profile of human and rodent theta class glutathione transferase T1-1

Background

The Theta class glutathione transferase GST T1-1 is a ubiquitously occurring detoxication enzyme. The rat and mouse enzymes have high catalytic activities with numerous electrophilic compounds, but the homologous human GST T1-1 has comparatively low activity with the same substrates. A major structural determinant of substrate recognition is the H-site, which binds the electrophile in proximity to the nucleophilic sulfur of the second substrate glutathione. The H-site is formed by several segments of amino acid residues located in separate regions of the primary structure. The C-terminal helix of the protein serves as a lid over the active site, and contributes several residues to the H-site.

Methods

Site-directed mutagenesis of the H-site in GST T1-1 was used to create the mouse Arg234Trp for comparison with the human Trp234Arg mutant and the wild-type rat, mouse, and human enzymes. The kinetic properties were investigated with an array of alternative electrophilic substrates to establish substrate selectivity profiles for the different GST T1-1 variants.

Results

The characteristic activity profile of the rat and mouse enzymes is dependent on Arg in position 234, whereas the human enzyme features Trp. Reciprocal mutations of residue 234 between the rodent and human enzymes transform the substrate-selectivity profiles from one to the other.

Conclusions

H-site residue 234 has a key role in governing the activity and substrate selectivity profile of GST T1-1.

General significance

The functional divergence between human and rodent Theta class GST demonstrates that a single point mutation can enable or suppress enzyme activities with different substrates.     

Comparative studies on lipogenic enzyme activities in the liver of human and some animal species

1. The activities of enzymes involved in fatty acid synthesis in the human liver (sample taken during abdominal surgery) and in the livers of some animals were studied. 2. Fatty acid synthase, ATP-citrate lyase and malic enzyme activities were found to be from 4 to 70-fold lower in human liver than in rat or bird livers. 3. The activities of hexose monophosphate shunt dehydrogenases in human liver were from half to almost equal to the corresponding activities in birds, but much lower than in rat liver. 4. The activities of all enzymes listed above in human and beef liver were very similar (except fatty acid synthase which was undetectable in the beef liver). 5. Very high activity of NADP-linked isocitrate dehydrogenase was found in livers of all species tested. 6. These results are discussed in relation to the role of the human liver in lipogenesis. 7. The activities of the enzymes generating NADPH in human liver taken during abdominal surgery were similar to the activities observed in the tissue obtained post mortem. 8. This suggested that post mortem tissue may be used as a reliable human material for some enzyme assays. 9. Thus we also examined the activity of malic enzyme in post mortem human kidney cortex, heart, skeletal muscle and brain. 10. Relatively high activity of NADP-linked malic enzyme has been observed in human brain.     

PPARalpha agonists up-regulate organic cation transporters in rat liver cells

It has been shown that clofibrate treatment increases the carnitine concentration in the liver of rats. However, the molecular mechanism is still unknown. In this study, we observed for the first time that treatment of rats with the peroxisome proliferator activated receptor (PPAR)-alpha agonist clofibrate increases hepatic mRNA concentrations of organic cation transporters (OCTNs)-1 and -2 which act as transporters of carnitine into the cell. In rat hepatoma (Fao) cells, treatment with WY-14,643 also increased the mRNA concentration of OCTN-2. mRNA concentrations of enzymes involved in carnitine biosynthesis were not altered by treatment with the PPARalpha agonists in livers of rats and in Fao cells. We conclude that PPARalpha agonists increase carnitine concentrations in livers of rats and cells by an increased uptake of carnitine into the cell but not by an increased carnitine biosynthesis.     

Cytochrome P450(11β): Structure-function relationship of the enzyme and its involvement in blood pressure regulation

Cytochrome P450(11β) is deeply involved in the final steps of biosynthesis of mineralocorticoids. This paper deals with following issues about this enzyme. (1) The structure and function of the enzymes of various animal species are discussed. By making alignment of amino acid sequences of the enzymes, we identified peptide domains essential for the enzyme actions such as a putative steroid binding domain and a heme binding region. Estimates of molecular similarity among the P450(11β) family enzymes suggested that the enzymes having both 11β-hydroxylation activity and aldosterone (ALDO) synthetic activity of certain animals such as frog, cattle and pig are more similar to the ALDO synthases of the other animals, such as rat, mouse and human, than the 11β-hydroxylases of these animals. (2) The molecular nature of the P450(11β) family enzymes of genetically hypertensive rats as well as adrenal regeneration hypertension (ARH) rats is examined. (i) Mutation was found in the P450(11β) gene of Dahl's salt-resistant normotensive rat. Steroidogenic activity expressed by the mutated gene accounted well for abnormal plasma levels of steroid hormones in this rat. (ii) 11β-, 18- and 19-Hydroxylation activities of adrenal mitochondria prepared from spontaneously hypertensive rat (SHR), Wistar-Kyoto rat (WKY), and stroke-prone (SP)-SHR were not significantly different from each other. Levels of mRNA of ALDO synthase in adrenal glands of 50-week-old SHR was significantly lower than those of 10-week-old SHR, WKY and SHR-SP. (iii) No significant difference in 19-hydroxylation activity was found between adrenal mitochondria prepared from ARH rat and those from control rat. The level of message of ALDO synthase was lower in adrenal glands of ARH rat.     

Significance of renal gamma-butyrobetaine hydroxylase for carnitine biosynthesis in man

Carnitine biosynthesis was studied in man and rat. Three healthy adult men were given intravenous injections of 1 mCi of [methyl-3H]epsilon-N-trimethyl-L-lysine, a precursor of carnitine. Labeled metabolites of this compound were monitored in serum and urine at 2, 6, 12, 24, and 48 h. At least nine radioactive metabolites were detected. For each collecton period, the specific activity of urinary carnitine exceeded the average serum specific activity. In man, the amount of labeled carnitine in urine was 2 to 8 times greater than labeled gamma-butyrobetaine (the immediate precursor of carnitine). In similar experiments in rats (intravenous injection of 0.1 mCi of [methyl-3H]epsilon-N-trimethyl-L-lysine), the specific activity of carnitine in urine was always lower than the corresponding average specific activity in serum. Between 0 and 2 h after administration of labeled precursor, the animals excreted large amounts of labeled gamma-butyrobetaine but little labeled carnitine. Significant gamma-butyrobetaine, 2-oxoglutarate dioxygenase (EC 1.14.11.1) activity was found in human kidney but this activity was absent in rat kidney. The results indicate that in man and rat the kidney accumulates intravenously administered [methyl-3H]epsilon-N-trimethyl-L-lysine. This compound is metabolized predominantly to gamma-butyrobetaine in rat kidney and to carnitine in human kidney. In both species, the synthesized products are at least partially leaked (either by secretion or by passive diffusion down a concentration gradient) into the renal tubular lumen from which they are either reabsorbed into the circulation for distribution to other tissues or excreted.