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Kinetin — 45 Years On Plant Science 148 (1999) 37–45 www.elsevier.com/locate/plantsci Kinetin — 45 years on Jan Barciszewski a,*, Suresh I.S. Rattan b, Gunhild Siboska b, Brian F.C. Clark b a Institute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan, Poland b Department of Molecular and Structural Biology, Uni6ersity of Aarhus, DK-8000 Aarhus C, Denmark Received 23 November 1998; received in revised form 14 June 1999; accepted 15 June 1999 Abstract Kinetin (N6-furfuryladenine) was the first cytokinin to be isolated almost 45 years ago from DNA as an artifactual rearrangement product of the autoclaving process. Since then its chemical structure and properties have been well described. Most importantly, a wide variety of biological effects of kinetin, including those on gene expression, on inhibition of auxin action, on stimulation of calcium flux, on cell cycle, and as an anti-stress and anti-ageing compound have been reported. Recently, views on this very well known plant growth factor have undergone substantial modifications. New data have appeared which show that kinetin is formed in cellular DNA as the product of the oxidative, secondary modification of DNA. Although the biological significance of the endogenous kinetin and the molecular mechanisms of its action are not completely understood at present, most of the experimental data point toward kinetin acting as a strong antioxidant in vitro and in vivo, with potential beneficial uses in agriculture and human healthcare. © 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Cytokinin; Reactive oxygen species; Oxidative damage; Anti-ageing 1. Introduction tagonistic and additive interactions between them have been observed. Although several general re- Kinetin (N6-furfuryladenine) was the first cy- views on cytokinins have been published [3–6], we tokinin isolated and identified in 1955 [1,2]. Cy- consider that there are at least three main reasons tokinin is the generic name used to designate a which justify a review focusing entirely on kinetin plant-growth substance that promotes cell division as timely. These are: (i) the recent identification of and may play a role in cell differentiation. Most kinetin in cellular DNA and plant tissue extracts; commonly, cytokinins comprise a group of N6- (ii) the new data on the biological properties of substituted adenine derivatives that induce divi- kinetin; and (iii) the commercial applications of sion and organogenesis in plant cell cultures and kinetin in cosmeceuticals. Here we review data on affect other physiological and developmental pro- the structure, properties and mechanism of action cesses. However, despite almost a half century of of kinetin along with a discussion of the major study, virtually nothing has been revealed about issues yet to be resolved. the mechanisms that mediate a variety of re- sponses to cytokinins in general and kinetin in particular. There is also a view that the effects of 2. Structure, chemical properties and natural cytokinins are never exclusively the result of cy- occurrence tokinin action, but are rather co-mediated by other factors and hormones, since synergistic, an- N6-furfuryladenine or kinetin was isolated for the first time in 1955 from autoclaved herring * Corresponding author. Tel.: +48-61-8528-503, ext. 132; fax: +48-61-8520-532. sperm DNA and has been thought to be an artifi- E-mail address: [email protected] (J. Barciszewski) cial DNA rearrangement product [1,2]. As the 0168-9452/99/$ - see front matter © 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0168-9452(99)00116-8 38 J. Barciszewski et al. / Plant Science 148 (1999) 37–45 purine derivative (Fig. 1A) it can be easily split off refined to a R index of 0.06. The bond angles and DNA at slightly acidic conditions due to the abil- distances of adenine moiety are in good agreement ity of its glycosylic bond. Kinetin is an amphoteric with the corresponding values of the adenine with compound with pKa values 4 and 10. It is dis- exception of the C(6) (6) bond which is slightly solved in strong acids, alkalis, and glacial acetic longer in kinetin. The molecular conformation of acid, is slightly soluble in ethanol, butanol, ace- kinetin can be best characterised in term of two tone and ether, but is practically insoluble in dis- planes; one passing through the adenine moiety tilled water. It sublimes at 220°C at atmospheric and the other through the furfuryl group. The pressure and is unaffected by autoclaving either at dihedral angle between the two planes is 79°. Since pH 0.5 or 12 [2]. the N(6)-substituent is distal to the imidazole ring, Crystals of kinetin suitable for X-ray analysis hydrogen bonding of the Watson–Crick type is have been obtained by slow cooling of a hot not seen in crystals of the base. In contrast, the ethanol solution. The structure of kinetin was orientation of the substituent leads to the Hoogs- Fig. 1. N6-furfuryladenine or kinetin. (A) Chemical formula; (B) UV spectrum; and (C) mass spectrum. J. Barciszewski et al. / Plant Science 148 (1999) 37–45 39 teen type of base pairing across crystallographic kinetin in an extract of the root nodules of Ca- centres of inversion by N6H…N7 and suarina equisetifolia, produced by the inoculation N9H…N3 hydrogen bonds resulting in continu- of Frankia. Actinomyces that make up the genus ous ribbons of purine bases with the furfuryl ring Frankia are distinguished by their ability to induce pointing up and down from these sheets [7,8]. The N-fixing root nodules on certain non-leguminous distal orientation of the N6-substituent with re- plants [14]. Also palmarosa (Cymbopogon martinii spect to the imidazole ring is similar to other var. motia) roots contain 6-furfuryladenine, the substituted N6 adenines. An interesting conse- amount of which increased significantly after inoc- quence of Hoogsten type bonding is a hydrogen ulation of Glomus species [15]. bond interaction between C(8)H(8)…0(15) at The finding of kinetin in DNA and cell extracts 2.64 A,. Any substituent on N1 significantly re- raised the obvious question about its synthetic stricts conformational freedom of the N6CH2 pathway. Furfural has been suggested as a puta- bond because of the steric repulsion of these posi- tive precursor of kinetin [13]. It has been found tions [7–9]. that furfural is formed during hydroxyl radical A significant contribution to identification of oxidation of the C5% of deoxyribose in DNA kinetin in natural products was provided by dis- [16,17]. This aldehyde has also been found among covering its electrochemical properties [10]. This many of the reaction products of metallophor- gave the first approach for a chromatographic phyrins with DNA [18]. The C5% radical is formed analysis of kinetin in DNA and cell extracts using by H5% abstraction from the deoxyribose residue of an HPLC system with an electrochemical detector. DNA in addition to an oxidative attack at C1%, It is known that the basic components of nucleic which constitutes the principal mechanism of that acids are not electrochemically active at potentials reaction [17]. The calculated amount of furfural of about 300–700 mV. The electrochemical prop- (C5% hydroxylation reaction) relative to 5- erties of kinetin at about 650–900 mV potential methylene-2-furanone (C1% hydroxylation deriva- are due to the presence of the furfuryl substituent tive) catalysed by phorphirins was found to be at the exocyclic amine group of purine [10]. Simi- 15% and was suggested that furfural residue is one larly, the guanine residue after modification at C8 of the primary products of hydroxy radical dam- with the hydroxyl group (OH) acquires electro- age of DNA. chemical properties, which are currently used for Those mechanistic considerations have been identification of 8-oxodeoxyguanosine (8oxodG) based on the bleomycin mechanism or on product in DNA at the level of four residues of 8-oxo-dG analysis of DNA constituents that were treated per 106 deoxyguanosine residues [11]. with ionising radiation. Recently, deuterium ki- The crucial evidence for the presence of N6-fur- netic isotope effects on the rate of cleavage of furyladenine in natural products came from the DNA by the hydroxyl radical were measured. mass spectrometric analysis of DNA components These experiments demonstrate that the hydroxyl (Fig. 1C). The molecular signal of 215 m/e has radical reacts with the various hydrogen atoms of been identified and the pattern of mass signals the deoxyribose in the order 5%H\4%H\3%H= interpreted [10]. The spectra obtained for an iso- 2%H=1%H [19,20]. This order of reactivity not only lated product and that of N6-furfuryladenine al- parallels the exposure to solvent of the deoxyri- ready deposited in the mass spectra library at USA bose hydrogens, but also provides information on National Institute of Standards and Technology the mechanism of DNA damage. It is clear that were identical [12,13]. The presence of kinetin in the C5% hydroxylation and spontaneous cleavage DNA has been identified also by GC/MS analysis of the DNA backbone with b-elimination leads to of trimethylsilyl (TMS) derivatives of nucleic acids the furfural formation. The presence of furfural in bases, obtained after acidic hydrolysis of DNA. the extracts of various cells was confirmed by its Kinetin modified with one or two TMS groups reaction with 0-(2,3,4,5,6-pentafluorobenzyl) hy- showed the m/e signals of 287 and 359, respec- droxylamine hydrochloride [21,22]. The oxime tively [Dizdaroglu, personal communication]. Re- derivative obtained was then converted to TMS cently, new examples of kinetin’s presence in derivatives and analysed by mass spectrometry, natural products have been reported [14,15]. The which confirmed the furfural presence in the cell mass spectrometry and HPLC analyses showed extracts [23].
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