What are nucleotides needed for?
DNA and RNA synteshsis and energy transfer
ribose 5 phosphate for nculeotide syntehsis is derived from hexose monophopshate shunt
--acited by the addition of pyrophosphate from ATP, forming phosphoribosyl pyrophosphate (PRPP) using PRPP synthase
What are two way nucleotides are made?
de novo synthesis: in liver; purine and pyrimidines are synthesized from samller precusors and PRPP is added
salvage pathways: preforemd purine and pyrimidine based converted into nucleotides
--bases for salvage enzymes may arise from synthesis in liver and transport to other tisssues or digestion of endogenous nucleic acids (cell death, RNA turnover)
What is Lesch Nyhan disease?
enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent
salvage pathway is essential for adequate nucleotide syntehsis
CNS deterioration, mental retardation, spastic cerebral palsy assocaited with compulsive self-mutilation
cells in basal ganglia of brain (fine motor control) normally have high HPRT activity
hyperuricemia beacuase purines can't be salvaged
What are the steps of the nucleotide synthesis pathways?
How are pyrimidines synthesized?
synthesized de novo in cyotplasm from aspartate, CO2, glutamine
cytoplasmic carbamoyl phosphate synthetase
What is orotic aciduria?
severe anemia, megoblastic
urine contains crystalline residue--orotic aicd
treatment: uridine--bypassing defect in pyrimidine pathway
--salvaged to UMP which feedback inhibits carbamoyl phosphate synthase 2 preventing orotic acid formation
autosomal recessive disorder
defect in uridine monosphosphate (UMP) synthase: includes orotate phosphoribosyltransferase and orotidine decarboxylase
--lack of pyrimidines impairs nucleic acid syntehsis needed for hematopoiesis--megoblastic anemia)
oroti acid accumulates and spills in urine--orotic acid crystals and orotic acid urinary obstruction
What is the difference between the two orotic acidurias?
--no megaoblastic anemia
--pathway: urea cycle
--enzyme deficient: OTC
--pathway: pyrimidine synthesis
--enzyme deficient: UMP synthase
folate deficiency: megaloblastic anemia but no orotic aciduria
What are the steps of de novo pyrimidine sythesis?
What is the end product of pyrimidine synthesis?
conversion of UMP to dTMP
via ribonucleotide reductase, thymidylate synthase, and dihydrofolate reductase
--targest of antineoplastic drugs
What is cotrimoxazole?
contains synergistic antibiotics sulfamethoxazole and trimethoprim--inhibits different steps in prokaryotic syntehsis of tetrahydrofolate
What are the important enzymes of pyrimidine synthesis? What is their function and the drug that acts on them?
What is ribonucleotide reductase?
required for formation of deoxyribonucleotides for DNA synthesis
all four nucleotide substrates must be disphotes
dADP and dATP strongly inhibit ribonucleotide reductase
hydroxyurea--anticancer drug--blocks DNA synthesis indirectly by inhibiting ribonucleotide reductase
What is pyrimidine catabolism?
completely catabolized (NH+ is produced) or recycled by pyrimidine salvage enzymes
What is purine synthesis?
purines syntehsized de novo beginning with PRPP
most important enzyme: PRPP amidotransferase
catalyzes first and rate lmiting reaction of pathwya
--inhibited by three purine nculeotide end products: AMP, GMP, IMP
--drugs allopurinol (gout) and 6 mercaptopurine (antineoplastic) inhibit PRPP amidotransferase
--drugs are purine analogs that must be converted to nucleotides by HGPRT within cells
--amino acids glycine aspartate and glutamine are used in purine synthesis
--tetrahydrofolate required for syntehsis of all purines
--inosine monophosphate (contains purine base hypoxanthine) is precursor for AMP and GMP
How do parasites and protozoans make purines?
protzoan and multicellular parasites and many obligate parasites (Chlamydia) cna't syntehsize purine de novo becuase they lack necessary genes in purine pathway
-have elaborate salvage mechanismsfor acquiring purins from host to syntehsize their own nucleic acids to grow
What occurs with purine catabolism?
excess purine nucleotides or those released from DNA nad RNA by nculeases are cataoblzied first to nucleosides (loss of Pi) then to free purine bases (release of ribose or deoxyribose)
When do excess nucleoside monophosphates accumulate?
RNA is normally digested by nucleases (mRNAs and other types of RNAs are continuously turned over in nromal cells)
dying cells release DNA and RNA which is digested by nucleases
concentration of free Pi decerases as it may in galacosemia, hereditary fructose intolerance, and glucose 6 phosphatase deficiency
How important are salvage enzymes?
salvage enzymes recycle normally about 90% of purines, 10% are converted tourice acids and excreted in urine
when purine catabolism is increased significantly person is at risk for devleoping hyperuricemia and gout
What is adenosine deaminase deficiency (ADA)?
autosomal recessive disorder
produces severe combined immunodeficiency (SCID)
lacking both B and T cells functoin
children are multiply infected with many organisms (Pneumocystis carinii, Candida) and don't survive without treatment
treatment: enzyme replacement therapy and bone marrow transplant
high levels of dATP accumulate in red cells and inhibit ribonucleotide reductase--inhibiting production of other essential deoxynulceotide precursors for DNA synthesis
thought that impaired DNA synthesis contributes to dysfunction of B and T cells
What happens with treatment of large tumors?
treatment of large tumors with chemotherapeutic regimens or radiation presents excessive excretion of uric acid, reuslting in gout
cuase of excessive uric acid is destruction of cancer cell's nucleic acid into purines undergoing turnover
What is gout?
acute gouty arthritis
most often in males
results from precipitation of monosodium urate crystals in joints
crystals (negatively birefringent and needle shaped) initiate neutrophil mediated and acute inflammation (first affecting big tow)
chronic gout may manifest over time as tophi (deposits of monosodium urate) develop in soft tissue around joints, leading to chronic inflammation involving granulomas
--acute attacks of gout are treated wth colchicine or indomethacin to reduce inflammation
-chronic hyperuricemia becuase of underexcetion is treated with uricosuric drug (probenecid)
overproduction of uric acid and chronic gout are treated wtih allopurinol
What is hyperuricemia?
may be produced by overproduction of uric acid or underexcretion of uric acid by kidneys
may progress to acute and chronic gouty arthritis if uric acid (monosodium urate) is deposited in joints and surrounding soft tissues--where is causes inflammation
uric aid is produced from excess endogenous purines and from dietary purines (digestion of nucleic acid in intestine) by intestinal epithelial
--transported in blood to kidneys for excretion in the urine
What is the function of allopurinol?
inhibits xanthine oxidase and reduce purine syntehsis by inhibiting PRPP aminotranferase (provided HGPRT is active)
What conditions are accompanied by hyperuricemia and gout?
Lesch-Nyhan syndrome (no purine salvage)
partial deficiency of HGPRT
alcholism (lactate and urate compete for same transprot system inkidney
glucose 6 phosphatase deficiency
hereditary fructose intolerance
galactose 1 phosphate uridyl transferase deficiency (galactosemia)
--last 2 disease, phosporylated sugars accumulate decreasing the avialable Pi and increasing AMP (which can't be phosphorylated to ADP and ATP)--excess AMP is converted to uric acid
What is Lesch Nyhan syndrome?
x linked recessive conditoin
near complete deficiency of HGPRT activity
spastic cerebral palsy with compulsive biting of hands and lips
death often in first decade
--mutations in HGPRT gene on X chromosome: point mutation, complete deletion
--mutation results in increased Km for hypoxanthine and guanine for enzyme--mutation that cuase the encoded enzyme to have a short half life
tiny orange colored particles in diapers
uric acid in urine
--wihtout salvaging of hypoxanthine and guanine by HGRT, purines are shunted toward the excretion pathway
--compounded by lack of regulatory control of PRPP aminotransferase in purine sytnehsis pathway, resulting in sytheshsis of even more purines in body
large amounts of urate will cause crippling gouty arthritis and urate nephorphathy
renal failure is usually cause of death
treatment: allopurinol, ease amount of urate deposits formed