Putrescine

Last updated
Putrescine
Diaminobutane.svg
Putrescine-3D-balls.png
Names
Preferred IUPAC name
Butane-1,4-diamine
Other names
1,4-Diaminobutane, 1,4-Butanediamine
Identifiers
3D model (JSmol)
3DMet
605282
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.003.440 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 203-782-3
1715
KEGG
MeSH Putrescine
PubChem CID
RTECS number
  • EJ6800000
UNII
UN number 2928
  • InChI=1S/C4H12N2/c5-3-1-2-4-6/h1-6H2 Yes check.svgY
    Key: KIDHWZJUCRJVML-UHFFFAOYSA-N Yes check.svgY
  • NCCCCN
Properties
C4H12N2
Molar mass 88.154 g·mol−1
AppearanceColourless crystals
Odor fishy-ammoniacal, pungent
Density 0.877 g/mL
Melting point 27.5 °C (81.5 °F; 300.6 K)
Boiling point 158.6 °C; 317.4 °F; 431.7 K
Miscible
log P −0.466
Vapor pressure 2.33 mm Hg at 25 deg C (est)
3.54x10−10 atm-cu m/mol at 25 deg C (est)
1.457
Hazards
GHS labelling:
GHS-pictogram-flamme.svg GHS-pictogram-acid.svg GHS-pictogram-skull.svg
Danger
H228, H302, H312, H314, H331
P210, P261, P280, P305+P351+P338, P310
Flash point 51 °C (124 °F; 324 K)
Explosive limits 0.98–9.08%
Lethal dose or concentration (LD, LC):
  • 463 mg kg−1(oral, rat)
  • 1.576 g kg−1(dermal, rabbit)
Related compounds
Related alkanamines
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Putrescine is an organic compound with the formula (CH2)4(NH2)2. It is a colorless solid that melts near room temperature. It is classified as a diamine. [3] Together with cadaverine, it is largely responsible for the foul odor of putrefying flesh, but also contributes to other unpleasant odors.

Contents

Production

Putrescine is produced on an industrial scale by the hydrogenation of succinonitrile. [3]

Biotechnological production of putrescine from a renewable feedstock has been investigated. A metabolically engineered strain of Escherichia coli that produces putrescine at high concentrations in glucose mineral salts medium has been described. [4]

Biochemistry

Biosynthesis of spermidine and spermine from putrescine. Ado = 5'-adenosyl. Polyamine synthesis.svg
Biosynthesis of spermidine and spermine from putrescine. Ado = 5'-adenosyl.

Spermidine synthase uses putrescine and S-adenosylmethioninamine (decarboxylated S-adenosyl methionine) to produce spermidine. Spermidine in turn is combined with another S-adenosylmethioninamine and gets converted to spermine.

Putrescine is synthesized in small quantities by healthy living cells by the action of ornithine decarboxylase.

Putrescine is synthesized biologically via two different pathways, both starting from arginine.

Putrescine, via metabolic intermediates including N-acetylputrescine, γ-aminobutyraldehyde (GABAL), N-acetyl-γ-aminobutyric acid (N-acetyl-GABAL), and N-acetyl-γ-aminobutyric acid (N-acetyl-GABA), biotransformations mediated by diamine oxidase (DAO), monoamine oxidase B (MAO-B), aminobutyraldehyde dehydrogenase (ABALDH), and other enzymes, can act as a minor biological precursor of γ-aminobutyric acid (GABA) in the brain and elsewhere. [6] [7] [8] [9] [10] [11] In 2021, it was discovered that MAO-B does not mediate dopamine catabolism in the rodent striatum but instead participates in striatal GABA synthesis and that synthesized GABA in turn inhibits dopaminergic neurons in this brain area. [12] [11] It has been found that MAO-B, via the putrescine pathway, importantly mediates GABA synthesis in astrocytes in various brain areas, including in the hippocampus, cerebellum, striatum, cerebral cortex, and substantia nigra pars compacta (SNpc). [12] [11]

Occurrence

Putrescine is found in all organisms. [13] Putrescine is widely found in plant tissues, [13] often being the most common polyamine present within the organism. Its role in development is well documented, but recent studies have suggested that putrescine also plays a role in stress responses in plants, both to biotic and abiotic stressors. [14] The absence of putrescine in plants is associated with an increase in both parasite and fungal population in plants.

Putrescine serves an important role in a multitude of ways, which include: a cation substitute, an osmolyte, or a transport protein. [13] It also serves as an important regulator in a variety of surface proteins, both on the cell surface and on organelles, such as the mitochondria and chloroplasts. A recorded increase of ATP production has been found in mitochondria and ATP synthesis by chloroplasts with an increase in mitochondrial and chloroplastic putrescine, but putrescine has also been shown to function as a developmental inhibitor in some plants, which can be seen as dwarfism and late flowering in Arabiadopsis plants. [13]

Putrescine production in plants can also be promoted by fungi in the soil. [15] Piriformospora indica (P. indica) is one such fungus, found to promote putrescine production in Arabidopsis and common garden tomato plants. In a 2022 study it was shown that the presence of this fungus had a promotional effect on the growth of the root structure of plants. After gas chromatography testing, putrescine was found in higher amounts in these root structures. [16]

Plants that had been inoculated with P. indica had presented an excess of arginine decarboxylase. [16] This is used in the process of making putrescine in plant cells. One of the downstream effects of putrescine in root cells is the production of auxin. That same study found that putrescine added as a fertilizer showed the same results as if it was inoculated with the fungus, which was also shown in Arabidopsis and barley. The evolutionary foundations of this connection and putrescine are still unclear.

Putrescine is a component of bad breath and bacterial vaginosis. [17] It is also found in semen and some microalgae, together with spermine and spermidine.

Uses

Putrescine reacts with adipic acid to yield the polyamide nylon 46, which is marketed by Envalior (formerly DSM) under the trade name Stanyl. [18] [19]

Application of putrescine, along with other polyamines, can be used to extend the shelf life of fruits by delaying the ripening process. [20] Pre-harvest application of putrescine has been shown to increase plant resistance to high temperatures and drought. [21] Both of these effects seem to result from lowered ethylene production following exogenous putrescine exposure. [22]

Due to its role in putrification, putrescine has also been proposed as a biochemical marker for determining how long a corpse has been decomposing. [23]

Putrescine together with chitosan has been successfully used in postharvest physiology as a natural fruit coating. [24] Putrescine with chitosan treated fruits had higher antioxidant capacity and enzyme activities than untreated fruits. Fresh strawberries coated have lower decay percentage, higher tissue firmness, contents of total soluble solids. Nanoparticles of putrescine with chitosan are effective in preserving the nutritional quality and prolonging the post-harvest life of strawberries during storage up to 12 days. [24]

History

Putrescine and cadaverine were first described in 1885 by the Berlin physician Ludwig Brieger (1849–1919). [25] [26] [27]

Toxicity

In rats, putrescine has a low acute oral toxicity of 2000 mg/kg body weight, with no-observed-adverse-effect level of 2000 ppm (180 mg/kg body weight/day). [28]

Further reading

Related Research Articles

<span class="mw-page-title-main">Cadaverine</span> Foul-smelling diamine compound

Cadaverine is an organic compound with the formula (CH2)5(NH2)2. Classified as a diamine, it is a colorless liquid with an unpleasant odor. It is present in small quantities in living organisms but is often associated with the putrefaction of animal tissue. Together with putrescine, it is largely responsible for the foul odor of putrefying flesh, but also contributes to other unpleasant odors.

<span class="mw-page-title-main">Arginine</span> Amino acid

Arginine is the amino acid with the formula (H2N)(HN)CN(H)(CH2)3CH(NH2)CO2H. The molecule features a guanidino group appended to a standard amino acid framework. At physiological pH, the carboxylic acid is deprotonated (−CO2−) and both the amino and guanidino groups are protonated, resulting in a cation. Only the l-arginine (symbol Arg or R) enantiomer is found naturally. Arg residues are common components of proteins. It is encoded by the codons CGU, CGC, CGA, CGG, AGA, and AGG. The guanidine group in arginine is the precursor for the biosynthesis of nitric oxide. Like all amino acids, it is a white, water-soluble solid.

<span class="mw-page-title-main">Ornithine decarboxylase</span>

The enzyme ornithine decarboxylase catalyzes the decarboxylation of ornithine to form putrescine. This reaction is the committed step in polyamine synthesis. In humans, this protein has 461 amino acids and forms a homodimer.

Agmatine, also known as 4-aminobutyl-guanidine, was discovered in 1910 by Albrecht Kossel. It is a chemical substance which is naturally created from the amino acid arginine. Agmatine has been shown to exert modulatory action at multiple molecular targets, notably: neurotransmitter systems, ion channels, nitric oxide (NO) synthesis, and polyamine metabolism and this provides bases for further research into potential applications.

<span class="mw-page-title-main">Glutamate decarboxylase</span> Enzyme

Glutamate decarboxylase or glutamic acid decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of glutamate to gamma-aminobutyric acid (GABA) and carbon dioxide. GAD uses pyridoxal-phosphate (PLP) as a cofactor. The reaction proceeds as follows:

Spermine is a polyamine involved in cellular metabolism that is found in all eukaryotic cells. The precursor for synthesis of spermine is the amino acid ornithine. It is an essential growth factor in some bacteria as well. It is found as a polycation at physiological pH. Spermine is associated with nucleic acids and is thought to stabilize helical structure, particularly in viruses. It functions as an intracellular free radical scavenger to protect DNA from free radical attack. Spermine is the chemical primarily responsible for the characteristic odor of semen.

<span class="mw-page-title-main">Spermidine</span> Chemical compound

Spermidine is a polyamine compound found in ribosomes and living tissues and having various metabolic functions within organisms.

<span class="mw-page-title-main">SpeF leader</span> Cis-acting element

SpeF is a putative cis-acting element identified in several gram negative alpha proteobacteria. It is proposed to be involved in regulating expression of genes involved in polyamide biosynthesis.

A ureohydrolase is a type of hydrolase enzyme. The ureohydrolase superfamily includes arginase, agmatinase, formiminoglutamase and proclavaminate amidinohydrolase. These enzymes share a 3-layer alpha-beta-alpha structure, and play important roles in arginine/agmatine metabolism, the urea cycle, histidine degradation, and other pathways.

<span class="mw-page-title-main">Biosynthesis of cocaine</span>

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4-(γ-Glutamylamino)butanoic acid is molecule that consists of L-glutamate conjugated to γ-aminobutyric acid (GABA). It is the substrate of the enzyme γ-glutamyl-γ-aminobutyrate hydrolase, which is involved in the biosynthesis of polyamines.

A polyamine is an organic compound having more than two amino groups. Alkyl polyamines occur naturally, but some are synthetic. Alkylpolyamines are colorless, hygroscopic, and water soluble. Near neutral pH, they exist as the ammonium derivatives. Most aromatic polyamines are crystalline solids at room temperature.

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The ldcC RNA motif is a conserved RNA structure that was discovered by bioinformatics. ldcC motif RNAs are found in Bacillota and two species of Spirochaetota.

γ-Aminobutyraldehyde, also known as 4-aminobutanal, 4-aminobutyraldehyde, or GABA aldehyde, is a metabolite of putrescine and a biological precursor of γ-aminobutyric acid (GABA). It can be converted into GABA by the actions of diamine oxidase (DAO) and aminobutyraldehyde dehydrogenase (ABALDH). Putrescine is converted into ABAL via monoamine oxidase B (MAO-B). However, biosynthesis of GABA from polyamines like putrescine is a minor metabolic pathway in the brain.

4-Amino-1-butanol, or 4-aminobutanol, also known as 4-hydroxybutylamine, is an alkanolamine and an analogue and precursor of the neurotransmitter γ-aminobutyric acid (GABA).

<span class="mw-page-title-main">Neurotransmitter prodrug</span> A prodrug of a neurotransmitter

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<i>N</i>-Acetylputrescine Endogenous GABA precursor

N-Acetylputrescine (NacPut), also known as monoacetylputrescine, is an endogenous metabolite of putrescine and a precursor and metabolic intermediate in the biosynthesis of γ-aminobutyric acid (GABA) from putrescine.

<i>N</i>-Acetyl-γ-aminobutyric acid Endogenous GABA precursor

N-Acetyl-γ-aminobutyric acid (N-acetyl-GABA), also known as N-acetyl-4-aminobutyric acid, is a metabolic intermediate in the biosynthesis of γ-aminobutyric acid (GABA) from putrescine. Other intermediates in this pathway include N-acetylputrescine and N-acetyl-γ-aminobutyraldehyde (N-acetyl-GABAL or N-acetyl-GABA aldehyde). Enzymes mediating the transformations in this pathway include putrescine acetyltransferase (PAT), monoamine oxidase B (MAO-B), aldehyde dehydrogenase (ALDH), and an unknown deacetylase enzyme. The pathway is a minor pathway in GABA synthesis compared to the main pathway in which GABA is synthesized from glutamate. However, the pathway has been found to have an important physiological role in the brain, for instance in the production of GABA in the striatum and resultant inhibition of dopaminergic neurons in this brain area.

<i>N</i>-Acetyl-γ-aminobutyraldehyde Endogenous GABA precursor

N-Acetyl-γ-aminobutyraldehyde (N-acetyl-GABAL), also known as N-acetyl-GABA aldehyde, is a metabolic intermediate in the biosynthesis of γ-aminobutyric acid (GABA) from putrescine. Other intermediates in this pathway include N-acetylputrescine and N-acetyl-γ-aminobutyric acid (N-acetyl-GABA). Enzymes mediating the transformations in this pathway include putrescine acetyltransferase (PAT), monoamine oxidase B (MAO-B), aldehyde dehydrogenase (ALDH), and an unknown deacetylase enzyme. The pathway is a minor pathway in GABA synthesis compared to the main pathway in which GABA is synthesized from glutamate. However, the pathway has been found to have an important physiological role in the brain, for instance in the production of GABA in the striatum and resultant inhibition of dopaminergic neurons in this brain area.

References

  1. ↑ Thalladi, V.R.; Boese, R.; Weiss, H.-C. (2001). "CSD Entry: QATWAJ : 1,4-Butanediamine". Cambridge Structural Database: Access Structures. Cambridge Crystallographic Data Centre. doi:10.5517/cc4g850 . Retrieved 2021-11-07.
  2. ↑ Thalladi, V. R.; Boese, R.; Weiss, H.-C. (2000). "The Melting Point Alternation in α,ω-Alkanediols and α,ω-Alkanediamines: Interplay between Hydrogen Bonding and Hydrophobic Interactions". Angew. Chem. Int. Ed. 39 (5): 918–922. doi:10.1002/(SICI)1521-3773(20000303)39:5<918::AID-ANIE918>3.0.CO;2-E. PMID   10760893.
  3. 1 2 Eller, Karsten; Henkes, Erhard; Rossbacher, Roland; Höke, Hartmut (2000). "Amines, Aliphatic". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a02_001. ISBN   3527306730.
  4. ↑ Qian, Zhi-Gang; Xia, Xiao-Xia; Yup Lee, Sang (2009). "Metabolic Engineering of Escherichia coli for the Production of Putrescine: A Four Carbon Diamine". Biotechnology and Bioengineering. 104 (4): 651–662. doi: 10.1002/bit.22502 . PMID   19714672.
  5. ↑ Srivenugopal KS, Adiga PR (September 1981). "Enzymic conversion of agmatine to putrescine in Lathyrus sativus seedlings. Purification and properties of a multifunctional enzyme (putrescine synthase)". J. Biol. Chem. 256 (18): 9532–41. doi: 10.1016/S0021-9258(19)68795-8 . PMID   6895223.
  6. ↑ Rashmi, Deo; Zanan, Rahul; John, Sheeba; Khandagale, Kiran; Nadaf, Altafhusain (2018). "γ-Aminobutyric Acid (GABA): Biosynthesis, Role, Commercial Production, and Applications". Studies in Natural Products Chemistry. Vol. 57. Elsevier. pp. 413–452. doi:10.1016/b978-0-444-64057-4.00013-2. ISBN   978-0-444-64057-4. Alternate pathways of GABA synthesis from putrescine and other polyamines have also been reported [207–211]. Here, γ-aminobutyraldehyde, an intermediate from polyamine degradation reaction via combined activities of diamine oxidase (DAO, E.C. 1.4.3.6) and 4-aminobutyraldehyde dehydrogenase (ABALDH), leads to the synthesis of GABA [205,212,213]. In response to abiotic stresses, GABA is also reported to be synthesized from proline via D1-pyrroline intermediate formation [47,205,214] and also by a nonenzymatic reaction [214]. However, GABA synthesis from polyamine pathways is minor in the brain, [215] although they play a significant role in the developing brain [216] and retina [217]. But GABA can be formed from putrescine in the mammalian brain [218].
  7. ↑ Shelp BJ, Bozzo GG, Trobacher CP, Zarei A, Deyman KL, Brikis CJ (September 2012). "Hypothesis/review: contribution of putrescine to 4-aminobutyrate (GABA) production in response to abiotic stress". Plant Sci. 193–194: 130–135. Bibcode:2012PlnSc.193..130S. doi:10.1016/j.plantsci.2012.06.001. PMID   22794926.
  8. ↑ Benedetti MS, Dostert P (1994). "Contribution of amine oxidases to the metabolism of xenobiotics". Drug Metab Rev. 26 (3): 507–535. doi:10.3109/03602539408998316. PMID   7924902. MAO also catalyses the deamination of a natural brain constituent, monoacetyl-putrescine, producing y-acetylaminobutyraldehyde, which in turn participates in the formation of brain GABA [13].
  9. ↑ Watanabe M, Maemura K, Kanbara K, Tamayama T, Hayasaki H (2002). "GABA and GABA Receptors in the Central Nervous System and Other Organs". A Survey of Cell Biology. International Review of Cytology. Vol. 213. pp. 1–47. doi:10.1016/s0074-7696(02)13011-7. ISBN   978-0-12-364617-0. PMID   11837891.
  10. ↑ Seiler N (June 2004). "Catabolism of polyamines". Amino Acids. 26 (3): 217–233. doi:10.1007/s00726-004-0070-z. PMID   15221502.
  11. 1 2 3 Cho HU, Kim S, Sim J, Yang S, An H, Nam MH, Jang DP, Lee CJ (July 2021). "Redefining differential roles of MAO-A in dopamine degradation and MAO-B in tonic GABA synthesis". Exp Mol Med. 53 (7): 1148–1158. doi:10.1038/s12276-021-00646-3. PMC   8333267 . PMID   34244591.
  12. 1 2 Nam MH, Sa M, Ju YH, Park MG, Lee CJ (April 2022). "Revisiting the Role of Astrocytic MAOB in Parkinson's Disease". Int J Mol Sci. 23 (8): 4453. doi: 10.3390/ijms23084453 . PMC   9028367 . PMID   35457272.
  13. 1 2 3 4 Cui, Jing; Pottosin, Igor; Lamade, Emmanuelle; Tcherkez, Guillaume (June 2020). "What is the role of putrescine accumulated under potassium deficiency?". Plant, Cell & Environment. 43 (6): 1331–1347. doi:10.1111/pce.13740. ISSN   0140-7791. PMID   32017122. S2CID   211023002.
  14. ↑ González-Hernández, Ana Isabel; Scalschi, Loredana; Vicedo, Begonya; Marcos-Barbero, Emilio Luis; Morcuende, Rosa; Camañes, Gemma (January 2022). "Putrescine: A Key Metabolite Involved in Plant Development, Tolerance and Resistance Responses to Stress". International Journal of Molecular Sciences. 23 (6): 2971. doi: 10.3390/ijms23062971 . ISSN   1422-0067. PMC   8955586 . PMID   35328394.
  15. ↑ Copeland, Charles (2022-04-01). "The feeling is mutual: Increased host putrescine biosynthesis promotes both plant and endophyte growth". Plant Physiology. 188 (4): 1939–1941. doi:10.1093/plphys/kiac001. ISSN   0032-0889. PMC   8968283 . PMID   35355052.
  16. 1 2 Ioannidis, Nikolaos E.; Cruz, Jeffrey A.; Kotzabasis, Kiriakos; Kramer, David M. (2012-01-12). "Evidence That Putrescine Modulates the Higher Plant Photosynthetic Proton Circuit". PLOS ONE. 7 (1): e29864. Bibcode:2012PLoSO...729864I. doi: 10.1371/journal.pone.0029864 . ISSN   1932-6203. PMC   3257247 . PMID   22253808.
  17. ↑ Yeoman, CJ; Thomas, SM; Miller, ME; Ulanov, AV; Torralba, M; Lucas, S; Gillis, M; Cregger, M; Gomez, A; Ho, M; Leigh, SR; Stumpf, R; Creedon, DJ; Smith, MA; Weisbaum, JS; Nelson, KE; Wilson, BA; White, BA (2013). "A multi-omic systems-based approach reveals metabolic markers of bacterial vaginosis and insight into the disease". PLOS ONE. 8 (2): e56111. Bibcode:2013PLoSO...856111Y. doi: 10.1371/journal.pone.0056111 . PMC   3566083 . PMID   23405259.
  18. ↑ "Stanyl®". DSM. Archived from the original on 25 September 2017.
  19. ↑ "PA46 - Stanyl®". Envalior . Retrieved 28 August 2024.
  20. ↑ Abbasi, Nadeem Akhtar; Ali, Irfan; Hafiz, Ishfaq Ahmad; Alenazi, Mekhled M.; Shafiq, Muhammad (January 2019). "Effects of Putrescine Application on Peach Fruit during Storage". Sustainability. 11 (7): 2013. doi: 10.3390/su11072013 .
  21. ↑ Todorov, D.; Alexieva, V.; Karanov, E. (1998-12-01). "Effect of Putrescine, 4-PU-30, and Abscisic Acid on Maize Plants Grown under Normal, Drought, and Rewatering Conditions". Journal of Plant Growth Regulation. 17 (4): 197–203. doi:10.1007/PL00007035. ISSN   1435-8107. PMID   9892742. S2CID   20062811.
  22. ↑ Khan, A.S.; Z. Singh (May 2008). "Influence of Pre and Postharvest Applications of Putrescine on Ethylene Production, Storage Life and Quality of 'Angelino' Plum". Acta Horticulturae (768): 125–133. doi:10.17660/ActaHortic.2008.768.14. ISSN   0567-7572.
  23. ↑ Pelletti, Guido; Garagnani, Marco; Barone, Rossella; Boscolo-Berto, Rafael; Rossi, Francesca; Morotti, Annalisa; Roffi, Raffaella; Fais, Paolo; Pelotti, Susi (2019-04-01). "Validation and preliminary application of a GC–MS method for the determination of putrescine and cadaverine in the human brain: a promising technique for PMI estimation". Forensic Science International. 297: 221–227. doi:10.1016/j.forsciint.2019.01.025. ISSN   0379-0738. PMID   30831414. S2CID   73461335.
  24. 1 2 Bahmani, R; Razavi, F; Mortazavi, S; Juárez-Maldonado, A; Gohari, G (February 2024). "Chitosan–putrescine nanoparticle coating attenuates postharvest decay and maintains ROS scavenging system activity of strawberry cv. 'Camarosa' during cold storage". Folia Horticulturae . 36 (1). Polish Society of Horticultural Science: 149–160. doi: 10.2478/fhort-2024-0009 . S2CID   19887643.
  25. ↑ Brief biography of Ludwig Brieger Archived 2011-10-03 at the Wayback Machine (in German). Biography of Ludwig Brieger in English.
  26. ↑ Ludwig Brieger, "Weitere Untersuchungen über Ptomaine" [Further investigations into ptomaines] (Berlin, Germany: August Hirschwald, 1885), page 43. From page 43: Ich nenne dasselbe Putrescin, von putresco, faul werden, vermodern, verwesen. (I call this [compound] "putrescine", from [the Latin word] putresco, to become rotten, decay, rot.)
  27. ↑ Ludwig Brieger, "Weitere Untersuchungen über Ptomaine" [Further investigations into ptomaines] (Berlin, Germany: August Hirschwald, 1885), page 39.
  28. ↑ Til, H.P.; Falke, H.E.; Prinsen, M.K.; Willems, M.I. (1997). "Acute and subacute toxicity of tyramine, spermidine, spermine, putrescine and cadaverine in rats". Food and Chemical Toxicology. 35 (3–4): 337–348. doi:10.1016/S0278-6915(97)00121-X. ISSN   0278-6915. PMID   9207896.
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