Purines and Their Metabolism in Plants

In addition to the bound purine moieties of DNA and RNA, plants and animals contain a variety of other purine derivatives ranging in decreasing structural complexity from free nucleotides to simple substituted bases. Many of these compounds play important roles in metabolism and in its control and integration. Whereas most of the purine derivatives found in animal tissues also occur in plants, a number of naturally occurring purine derivatives are peculiar to plants. The most common purine compounds in Nature are those derived from the hydroxyand amino-substituted bases, probably the most widespread being those related to adenine. Adenine itself, like guanine and other purine bases, is alleged to occur in the free state in a large variety of plant species (see, e.g., Wehmer & Hadders, 1933; Karrer, 1958). Uric acid has been reported as a constituent of seeds of legumes (Fosse et a[., 1932). However, it is difficult to assess the value of many of these early reports, as the extraction methods used frequently led to chemical or enzymic degradation of nucleic acids and nucleotides. Further, as Markham (1955) pointed out, the methods used have not always led to unequivocal identification. Incontrast withsome of these earlier reports, later work with plant tissues failed to show detectable traces of free purine bases (see, e.g., Brown, 1963). This is not to say that they do not occur in plants, for they may have a transitory existence as metabolic intermediates. It is the evidence for their accumulation that needs re-appraisal. In contrast with free purine bases, free pyrimidine bases have been seldom reported to occur in plants. This, it may be suspected, is because of the greater resistance of pyrimidine nucleotides to acid hydrolysis and hence to the drastic acid-extraction procedures formerly in use. Probably the most often cited example of the occurrence of free purine bases in plants is that of the methylated xanthines in tea and coffee. The corresponding N-ribosides and N-ribotides are unknown, and, in the case of caffeine, further N-substitution would not be possible. This suggests that methylated xanthines are not artifacts resulting from hydrolysis of more complex purine compounds. Within recent years a number of purine derivatives with side chains not obviously related to ribose, or any other carbohydrate, have been found in higher plants. An unusual adenine derivative of this type, isolated from leaves and flowers of Hohrrhenafloribunda and Chidlovia sunguinea and from leaves of the legume Gladitschia triacanthos, has been identified as 3-(isopent-2-enyl)adenine (Leonard & Deyrup, 1962). This substance, known as ‘triacanthine’, is isomeric with N6-(isopent-2-enyl)adenine isolated from the plant pathogen Corynebacterium fascians and from hydrolysis of RNA derived from yeast and rat liver. A related compound, obtained from kernels of sweet corn and identified as N6-(truns-4-hydroxy-3-methylbut-2-enyl)adenine, was given the name ‘zeatin’ (Letham, 1963). It possesses strong cytokinin activity, as do a series of related substituted adenines discovered in plants subsequently and that include the riboside and the ribotide of zeatin (Letham, 1966a,b). It would again appear likely that hydrolysis during extraction accounts for the free bases. Several other purine ribosides, including adenosine and guanosine, are found in plants (see, e.g., Brown, 1963). Some are, however, derivatives of less common bases such as isoguanine (6-amino-2-hydroxypurine), which occurs in croton beans (Croton tiglium) as the 9-B-~-ribofuranoside, crotonside (Cherbuliez & Bernhard, 1932). Nebularine, an antibiotic produced by the fungus Clitocybe nebuluris, is of biochemical interest as the 9-fl-~-ribofuranoside of purine. Purine glycosides containing sugars other than ribose or 2-deoxyribose are also found in plants, mostly fungi. For example, puromycin, an antibiotic purine glycoside produced by Streptomyces alboniger, is an adenine derivative containing an amino sugar moiety, namely D-3-amino-3-deoxyribose. The same sugar occurs in the purine glycoside anti-