**Abstract**

Analyses in non-aspirin takers finding salicylic acid (SA) and hydroxylated metabolites in serum also SA and salicyluric acid (SU) in urine led to a re-evaluation of dietary sources of salicylates. Fruit and vegetable sources explained higher levels found in drugfree vegetarians, which overlapped with those from patients on low dose aspirin. That drug's chemo-protective action in cancer is, at least partially, attributable to its principal metabolite, SA—which we believe contributes to the benefits of a vegetarian diet. However, diet is unlikely to be the sole source of the circulating salicylate found in aspirin-free animals and man. We adduced evidence for its persistence in prolonged fasting and biosynthesis in vivo from labelled benzoic acid. We review the roles, defined and potential, of SA in the biosphere. Emphasis on the antiplatelet effect of aspirin in man has detracted from the likely pivotal role of SA in many potential areas of bioregulation—probably as important in animals as in plants. In this expanding field, some aspirin effects, mediated by apparently conserved receptors responding to SA, are discussed. The perspectives revealed may lead to re-evaluation of the place of salicylates in therapeutics and potentially improve formulations and drug delivery systems.

**Keywords:** salicylic acid, salicylic acid metabolism, dietary sources, biosynthesis, homologue receptors, conserved effects, siderophores, aspirin

#### **1. Introduction**

Salicylic acid (SA) in plants is a ubiquitous compound shown to be pivotal in initiating the response to a variety of physical, chemical and biological insults [1]. Analgesic and antipyretic properties of plant extracts, notably from willow, meadowsweet and myrtle, had been known for many centuries before isolation of SA as the active principle. Since synthesis of its acetyl ester—aspirin—investigation has focused on the properties of that compound which is rapidly hydrolysed (serum *t*1/2 of 20 min) to SA which itself has a half-life of 2–4 h [2]. The demonstration that aspirin works by serine side chain acetylation of Cox-1 and Cox-2 isoforms has detracted attention from SA itself; that compound, despite weak reversible Cox 1 and absent Cox 2 inhibition, is as effective as aspirin in vivo in suppressing inflammation [3]. Inhibition of transcription of the Cox-2 gene by micromolar concentrations of SA is the likely explanation [4].

### **2. SA without aspirin (SA sans ASA)**

Investigating the possible use of low dose aspirin as an aromatic probe to measure hydroxyl free radicals by assessing the hydroxylation of SA to form 2,3 and 2,5 dihydroxybenzoic acids (DHBAs)—**Figure 1**—required a sensitive HPLC assay with appropriate controls. That work revealed the presence of substances which had identical retention times to SA, 2,3 DHBA and 2,5 DHBA in the serum extracts of subjects not taking aspirin. The exclusion of contamination was followed by studies to determine the authenticity of these substances as SA, 2,3 DHBA and 2,5 DHBA.

*diet restricted to glucose and citric acid* [10]. Young reported a compound with thin layer chromatographic properties of SU [11] in a single individual on a synthetic diet, while Finnie and co-workers found a similar compound on their paper system in children not taking salicylate drugs [12]. At the time of all these earlier investigations methods for adequately characterising and quantifying the purported SA

Other work [13] designed to assess the dietary importance of salicylates had examined acid-treated urine using HPLC with fluorescence detection but salicylates other than SA, salicylate precursors and structurally related compounds may have

We examined 24 h urine samples from 10 volunteers who had not taken any salicylate drugs during the previous 2 weeks. The acid hydrophobic compounds (=organic acids) were separated using HPLC and quantified electrochemically. The Rts of the extracted substances and those of SA and SU were compared under two sets of chromatographic conditions and found very similar to those of authentic substances. The unknown substances, isolated by HPLC, and treated with acetyl chloride in methanol were compared with the methyl esters of SA and SU using GC–MS. After esterification the unknown compounds had mass spectra and Rts

These findings in serum and urine led us on to explore, in some detail, the likely dietary sources for the salicylate compounds we found and confirmed. At the time interest was re-awakening in the potential benefits of salicylates commonly found in the diet [13]—as opposed to what were considered traditional plant medicine

The occurrence of "natural salicylates", such as SA, in strawberries and other fruits was raised in the Lancet in 1903 [15] and the matter of whether these natural salicylates were superior to synthetic salicylates was the subject of a JAMA editorial in 1913 [16]—no superiority was concluded! Interest then appeared to wane until re-invigorated by the popularity, from 1970, in therapeutic trials—arising from the apparent cross-reactivity of tartrazine and aspirin—of exclusion diets.

Many plant derived non-nutritive compounds exert, in mammalian systems, biological activities that may have an impact on health and disease risk [17] and we proposed [18] that SA might provide a link between aspirin, diet and the prevention of colorectal cancer (CRC). There is good evidence that the regular intake of aspirin decreases the risk of developing cancer [19]. A mechanism for platelet mediated CRC tumorigenesis has been proposed [20]; that would, of course not be attributable to SA itself but a more balanced view is that both constituent groups of aspirin

Assessment of the extent of the contribution of diet to SA in blood and urine cannot be easily inferred from direct analysis of its concentration therein. There is considerable variability in peak serum levels of SA in subjects receiving a standard dose [7] while urinary salicylate is influenced by urine flow, pH, the presence of other organic acids and the saturability of SU formation and/or excretion [2].

(acetyl and SA moieties) contribute to the anti-cancer effects [21].

and SU were not readily available.

*Salicylic Acid Sans Aspirin in Animals and Man DOI: http://dx.doi.org/10.5772/intechopen.91706*

been included in the values reported.

**3. Dietary source of salicylates**

sources.

**3.1 Previously**

**3.2 Background**

**179**

comparable to those of methyl-SA and methyl-SU [14].
