Browsing Pathways

Betaine (or trimethylglycine) is similar to choline (trimethylaminoethanol) but differs in choline's terminal carboxylic acid group trimethylglycine is reduced to a hydroxyl group. Betaine is obtained from diet as betaine or compounds containing choline in foods such as whole grains, beets and spinach. Betaine can also be synthesized from choline in the liver and kidney. First, choline is oxidized to betaine aldehyde by mitochondrial choline oxidase (choline dehydrogenase). Then, betaine aldehyde dehydrogenase oxidizes betaine aldehyde to betaine in the mitochondria or cytoplasm. In the liver, betaine functions as a methyl donor similar to choline, folic acid, S-adenosyl methionine and vitamin B12. Methyl donors are important for liver function, cellular replication and detoxification reactions. Betaine is also involved in the production of carnitine to protect from kidney damage and functions as an osmoprotectant in the inner medulla.

The glycerol phosphate shuttle also known as the glycerophosphate shuttle. It shuttles electrons to mitochondrial carriers in the oxidative phosphorylation pathway from cytosolic NADH. This shuttle relies on mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH). This is also a common process for the cell to regenerate cytosolic NAD+ for other processes.

Phenylacetate (or phenylacetic acid) metabolism involves two steps. The first step is the conversion of phenylacetate into phenylacetyl-CoA which is catalyzed by acyl-coenzyme A synthetase ACSM1 or acyl-coenzyme A synthetase ACSM2B. Coenzyme A and ATP are also involved in this first step and AMP and pyrophosphate will be generated during the first step of metabolism. In the second step, phenylacetyl-CoA and L-glutamine interacts with glycine N-acyltransferase to generate coenzyme A as well as phenylacetylglutamine, of which the latter will be excreted in the urine. Phenylacetate metabolism provides a route that facilitates the excretion of nitrogen for patients with urea cycle defects; hence, it is important for clinical purposes.

The glucose-alanine cycle—also referred to in the literature as the Cahill cycle or the alanine cycle—involves muscle protein being degraded to provide more glucose to generate additional ATP for muscle contraction. It allows pyruvate and glutamate to be transported out of muscle tissue to the liver where gluconeogenesis takes place to supply the muscle tissue with more glucose as mentioned previously. To initiate the cycle, muscle and tissues that catabolize amino acids for fuel generate amino groups—most commonly in the form of glutamate—through the process of transamination. These amino groups are transferred via alanine aminotransferase to pyruvate (a product of glycolysis) to form alanine and alpha-ketoglutarate. Alanine subsequently moves through the circulatory system to the liver where the reaction previously catalyzed by alanine aminotransferase is reversed to produce pyruvate. This pyruvate is converted into glucose through the process of gluconeogenesis which subsequently is transported back to the muscle tissue. Meanwhile, glutamate dehydrogenase in the mitochondria catabolizes glutamate into ammonium. Ammonium moves on to form urea in the urea cycle.

Gluconeogenesis, which is essentially the reverse of glycolysis, results in the sythesis of glucose from non-carbohydrate substrates such as lactate, glycerol, and glucogenic amino acids. In animals, gluconeogenesis occurs primarily in the liver, and in the renal cortex to a lesser extent. This process occurs during periods of fasting or intense exercise. Gluconeogenesis is often associated with ketosis. Several non-carbohydrate carbon substrates can enter the gluconeogenesis pathway. One common substrate is lactic acid, formed during anaerobic respiration in skeletal muscle. Lactate may also come from red blood cells, which obtain energy solely from glycolysis as they have no membrane-bound organelles for aerobic respiration. Lactate is transported to the liver to be converted into pyruvate in the Cori cycle by lactate dehydrogenase. Pyruvate can then be used to generate glucose via gluconeogenesis. Many other compounds can also function as substrates for gluconeogenesis such as citric acid cycle intermediates (through conversion to oxaloacetate), amino acids other than lysine or leucine, and glycerol .

The malate-aspartate shuttle system, also called the malate shuttle, is an essential system used by mitochondria, that allows electrons to move across the impermeable membrane between the cytosol and the mitochondrial matrix. The electrons are created during glycolysis, and are needed for oxidative phosphorylation. The malate-aspartate shuttle is needed as the inner membrane is not permeable to NADH or NAD+, but is permeable to the ions that attach to malate. When the malate gets inside the membrane,the energy inside of malate is taken out by creating NADH from NAD+, which regenerates oxaloacetate. NADH can then transfer electrons to the electron transport chain.

Steroidogenesis is a process that through the transformations of other steroids, produces a desired steroid. Some of these desired steroids include cortisol, corticoids, testosterone, estrogens, aldosterone and progesterone. To begin the synthesis of steroid hormones, cholesterol synthesizes a hormone called pregnenolone. This is done by cholesterol from the cytosol or lysosome being brought to the mitochondria and becoming fixed in the inner mitochondrial membrane. Once there, the cholesterol becomes pregnenolone through three reactions. The enzyme responsible for catalyzing all three reactions is CYP11A, a side chain cleavage enzyme. After being created, the pregnenolone enters the cytosol, where the cholesterol originated. Once in the cytosol, pregenolone synthesizes progesterone, using two reactions. These two reactions are both catalyzed by an enzyme called 3-beta-hydroxysteroid dehydrogenase/isomerase. The enzyme CYP21A2 then hydroxylates progesterone, which converts it to deoxycorticosterone. Deoxycorticosterone then undergoes three reactions catalyzed by CYP11B2 to become aldosterone. 17alpha-hydroxyprogesterone is created from pregnenolone by using 3-beta-hydroxysteroid dehydrogenase/isomerase. CYP21A2 then hydroxylates 17alpha-hydroxyprogesterone which results in the production of 11-deoxycortisol. CYP11B1 quickly converts 11-deoxycortisol to cortisol. Cortisol is an active steroid hormone, and its conversion to the inactive cortisone has been known to occur in various tissues, with increased conversion occurring in the liver. Pregnenolone is an important hormone as it is responsible for the beginning of the synthesis of many hormones not pictured in this pathway such as testosterone and estrogen. Cortisol receptors are found in almost every bodily cell, so this hormone affects a wide range of body functions. Some of these functions include metabolism regulation, inflammation reduction, regulating blood sugar levels and blood pressure, and helps with the formation of memories.

Statins are a class of medications that lower lipid levels and are administered to reduce illness and mortality in people who are at high risk of cardiovascular disease. Atorvastatin (trade name: Lipitor) is a well-tolerated orally-administered synthetic statin that reduces levels of total cholesterol, low-density lipoprotein (LDL)-cholesterol, triglyceride, and very-low-density lipoprotein (VLDL)-cholesterol. It also increases levels of high-density lipoprotein (HDL)-cholesterol. Atorvastatin's efficacy is greater than other statins in reducing total cholesterol and LDL-cholesterol levels. This is theorized to be the result of a prolonged duration of HMG-CoA reductase inhibition. Reported side effects of atorvastatin include constipation, flatulence, dyspepsia (indigestion), abdominal pain, headache, and myalgia (muscle pain). The primary therapeutic mechanism of action of statins is the inhibition of the rate-limiting enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase in hepatocytes. HMG-CoA reductase catalyzes the conversion of HMG-CoA into mevalonic acid, a precursor for cholesterol biosynthesis. Statins bind reversibly to the active site of HMG-CoA reductase and the subsequent structural change in the enzyme effectively disables it. Due to the resulting decrease in intracellular sterol levels, the ER membrane protein INSIG no longer binds to SREBP cleavage-activating protein (SCAP) which is, itself, bound to the transcription factor sterol regulatory element-binding protein (SREBP). Freed from INSIG, SCAP escorts SREBP to the Golgi apparatus from the ER as cargo in COPII vesicles. At the Golgi membrane, two proteases, S1P and S2P, sequentially cleave the SCAP-SREBP complex, releasing the mature form of SREBP into the cytoplasm. SREBP then translocates to the nucleus where it is actively transported into the nucleoplasm by binding directly to importin beta in the absence of importin alpha. SREBP binds to the sterol regulatory element (SRE) present in the promoter region of genes involved in cholesterol uptake and cholesterol synthesis, including the gene encoding low-density lipoprotein (LDL) receptor (LDL-R). As a result, LDL-R gene transcription increases which then leads to an increased synthesis of the LDL-R protein. LDL-R localizes to the endoplasmic reticulum for transport and exocytosis to the cell surface. The elevated amount of LDL-R results in the binding of more circulating free LDL cholesterol and subsequent internalization via endocytosis. Lysosomal degradation of the internalized LDL cholesterol elevates cellular cholesterol levels to maintain homeostasis.

Triamterene is a diuretic that belongs to the potassium-sparing class of drugs which are commonly used to manage hypertension and edema. It acts by blocking epithelial sodium channels in the late distal convoluted tubule of the nephron. Specifically, triamterene inhibits amiloride-sensitive sodium channels which are responsible for the reabsorption of sodium in the late distal convoluted tubule in the nephron. This primarily contributes to an increase in sodium excretion and consequentially, fluid excretion which decreases blood volume and blood pressure. Potassium secretion is indirectly affected by the inhibition of sodium reabsorption due to the elimination of the electrochemical gradient that drives potassium loss. This leads to an increase in serum potassium concentration -- a common action for potassium-sparing drugs -- and has the potential to induce hyperkalemia which can potentially lead to severe heart arrhythmias.

Amiloride is a diuretic that inhibits the sodium channels in the late distal convoluted tubule and collecting tube of the nephron where 1-2% of sodium reabsorption occurs. The inhibition of sodium reabsorption results in increased osmolarity in the lumen and decreased osmolarity in the interstitium of the nephron. This decreased osmotic gradient results in a modest diuresis. The drug is also potassium sparing. Potassium is typically excreted due to the electrochemical gradient produced by sodium reabsorption. Therefore, amiloride's inhibition of sodium reabsorption fails to produce an electrochemical gradient and therefore, inhibits potassium excretion. Amiloride causes an increase in sodium excretion and a decrease in potassium secretion. The drug is typically prescribed to patients with depleted potassium.