Browsing Pathways

Mercaptopurine is a purine antimetabolite prodrug that exerts cytotoxic effects via three mechanisms: via incorporation of thiodeoxyguanosine triphosphate into DNA and thioguanosine triphosphate into RNA, inhibition of de novo synthesis of purine nucleotides, and inhibition of Ras-related C3 botulinum toxin substrate 1, which induces apoptosis of activated T cells. Mercaptopurine travels through the bloodstream and is transported into cells via nucleoside transporters. Mercaptopurine is then converted to thioguanosince diphosphate through a series of metabolic reactions that produces the metabolic intermediates, thioinosine 5’-monophosphate, thioxanthine monophosphate, and thioguanosine monophosphate. Thioguanosine diphosphate is then converted via a thiodeoxyguanosine diphosphate intermediate to thiodeoxyguanosine triphosphate, which is incorporated into DNA. Thioguanosine diphosphate is also converted to thioguanosine triphosphate which is incorporated into RNA. The thioguanosine triphosphate metabolite also inhibits Ras-related C3 botulinum toxin substrate 1, a plasma membrane-associated small GTPase that regulates cellular processes, inducing apoptosis in activated T cells. Finally, de novo synthesis of purine nucleotides is inhibited by the methyl-thioinosine 5’-monophosphate metabolite, which inhibits amidophosphoribosyl-transferase, the enzyme that catalyzes one of the first steps in this pathway.

Cyclophosphamide is an alkylating agent used in the treatment of certain cancers. Following absorption, cyclophosphamide is converted into 4-hydroxyphosphamide by a variety of cytochrome P450 isozymes in the liver. 4-Hydroxyphosphamide is more soluble than cyclophosphamide and is the primary form of the drug that is transported in blood. 4-Hydroxyphosphamide crosses the plasma membrane of the cancer cell and spontaneuosly forms aldophosphamide. This is a reversible reaction. Aldophosphamide can decompose into acrolein and phosphoramide mustard. Phosphoramide mustard is the active alkylating agent and forms alkyl adducts with DNA through a phosphoramide aziridinium intermediate. Alkylation of DNA causes DNA damage and eventually cell death.

Ifosfamide is an alkylating agent used in the treatment of certain cancers. Following absorption, ifosfamide is converted into 4-hydroxyifosfamide by a variety of cytochrome P450 isozymes in the liver. 4-Hydroxyifosfamide crosses the plasma membrane of the cancer cell and spontaneuosly forms aldoifosfamide. This is a reversible reaction. Aldoifosfamide can decompose into acrolein and ifosforamide mustard. Ifosforamide mustard is the active alkylating agent and forms alkyl adducts with DNA through an ifosforamide aziridinium intermediate. Alkylation of DNA causes DNA damage and ultimately cell death.

Opiate receptors are coupled with G-protein receptors and function as both positive and negative regulators of synaptic transmission via G-proteins that activate effector proteins. Binding of the opiate stimulates the exchange of GTP for GDP on the G-protein complex. As the effector system is adenylate cyclase and cAMP located at the inner surface of the plasma membrane, opioids decrease intracellular cAMP by inhibiting adenylate cyclase. Subsequently, the release of nociceptive neurotransmitters such as substance P, GABA, dopamine, acetylcholine and noradrenaline is inhibited. Opioids also inhibit the release of vasopressin, somatostatin, insulin and glucagon. Codeine's analgesic activity is, most likely, due to its conversion to morphine. Opioids close N-type voltage-operated calcium channels (OP2-receptor agonist) and open calcium-dependent inwardly rectifying potassium channels (OP3 and OP1 receptor agonist). This results in hyperpolarization and reduced neuronal excitability.

Methadone exerts its analgesic by acting on the mu-opioid receptor of sensory neurons. Binding to the mu-opioid receptor activates associated G(i) proteins. These subsequently act to inhibit adenylate cyclase, reducing the level of intracellular cAMP. G(i) also activates potassium channels and inactivates calcium channels causing the neuron to hyperpolarize. The end result is decreased nerve conduction and reduced neurotransmitter release, which blocks the perception of pain signals. Methadone further acts as an antagonist at the NMDA receptor, reducting calcium influx and neuronal excitability.

Prednisolone is a synthetic glucocorticoid that is used clinically for its anti-inflammatory properties. Prednisolone diffuses passively across the cell membrane, where it binds to glucocorticoid receptors in the cytoplasm. Upon binding, the glucocorticoid receptor (GR) dissociates from heat shock protein 90, and translocate into the nucleus. In the nucleus, GR dimers can bind to glucocorticoid response element (GRE) in the promoter region of anti-inflammatory genes, which activates their transcription. GRs also inhibit transcription of inflammatory mediators by binding to negative GRE (nGRE). GRs further interact with the transcription factors cAMP-responsive element binding protein and NF-kappa-B, and inihibit their activation of inflammatory gene transcription. GRs also recruit histone deacetylase 2 to inflammatory gene loci on DNA, which leads to DNA condensation and suppression of gene expression.

Capecitabine is a fluoropyrimidine anticancer drug. After absorption, it is metabolized in the liver to the intermediate 5’-deoxy-5-fluorouridine, which is subsequently converted into 5-fluorouracil (5-FU) by intracellular thymidine phosphorylase. 5-FU exerts cytotoxic effects on the cell by direct incorporation into DNA and RNA as well as by inhibiting thymidylate synthase. Since thymidine phosphorylase is present at 3-10 fold higher concentration in cancer cells compared normal cells, capecitabine’s cytotoxic effect is selective for cancer cells.

Adefovir dipivoxil is an ester prodrug of adefovir, a nucleotide analogue used in the treatment of chronic hepatitis B. Adefovir dipivoxil is taken up into the liver cell and is cleaved into adefovir by intracellular esterases. Adefovir is subsequently phosphorylated first by adenylate kinases and then by nucleoside diphosphate kinases into adefovir diphosphate. Adefovir diphosphate is an analogue of deoxyadenosine triphosphate (dATP) and competes with dATP for binding to the viral DNA polymerase and subsequent incorporation into the growing DNA strand. Once incorporated into the DNA, adefovir causes chain termination, thus preventing viral replication.

Ibuprofen is a very common NSAID drug used to treat pain and inflammation. This includes headaches, muscle pain and fever. It is sold under the brand name Advil or Motrin. Ibuprofen is typically ingested orally, although in the USA an intravenous version can be used. It inhibits cyclooxygenase (COX) non-selectively. This enzyme is responsible for the creation of prostaglandins, which allow pain to be felt. Inhibiting COX makes prostaglandin creation more sparse, thus resulting in less pain for the patient using ibuprofen. Arachdonic acid is converted into prostaglandin H2 by using cytosolic prostaglandin G/H synthase (COX). These enzymes are available as COX1 and COX2, and are encoded by PTGS1 (COX1) and PTGS2 (COX2). Ibuprofen may also inhibit fatty acid amide hydrolase (FAAH), which results in the activation of antinociceptive axis, which then metabolizes the endocannabinoid anandamide.

Biotinidase deficiency (Multiple carboxylase deficiency) is an autosomal recessive disease caused by a mutation in the BTD gene which codes for biotinidase. A deficiency in this enzyme results in accumulation of ammonia and ketone bodies in blood; 3-hydroxyisovaleric acid in plasma, spinal fluid, and urine; hydroxypropionic acid, 2-hydroxybutyric acid, 3-Hydroxybutyric acid, and citric acid in spinal fluid; and 3-methylcrotonylglycine, hydroxypropionic acid, and L and D-lactic acid in urine. Symptoms, which can present from birth into adulthood include hypotonia, ketosis, hyperammonemia, motor retardation, coma, and seborrhoic skin rash. Treatment includes biotin.