Sambunigrin, a diastereomer of prunasin derived from (S)-mandelonitrile instead of it the (R)-isomer, has been isolated from leaves of the elder tree (Sambucus nigra).[3] Sambunigrin is present in the leaves and stems of elder at a 1:3 ratio of sambunigrin to prunasin, and 2:5 in the immature seed.[4] It is not found in the root.[4]
Biosynthesis
Overview
(R)-prunasin begins with the common amino acid phenylalanine, which in plants is produced via the Shikimate pathway in primary metabolism. The pathway is catalyzed mainly by two cytochrome P450 (CYP) enzymes and a UDP-glucosyltransferase (UGT). After (R)-prunasin is formed, it is either converted into amygdalin by an additional UDP-glucosyltransferase or degraded into benzaldehyde and hydrogen cyanide.
Researchers have shown that the accumulation (or lack of) of prunasin and amygdalin in the almond kernel is responsible for sweet and bitter genotypes.[1] Because amygdalin is responsible for the bitter almond taste, almond growers have selected genotypes which minimize the biosynthesis of amygdalin. The CYP enzymes responsible for generation of prunasin are conserved across Prunus species.[5] There is a correlation between high concentration of prunasin in the vegetative regions of the plant and the sweetness of the almond, which is relevant to the almond agricultural industry. In almonds, the amygdalin biosynthetic genes are expressed at different levels in the tegument (mother tissue, or outer section) and cotyledon (kernel, or father tissue), and vary significantly during almond ontogeny.[1][6][7] The biosynthesis of prunasin occurs in the tegument, then transported to other tissues for conversion to amygdalin or degraded.[1][5]
Biosynthesis of (R)-prunasin
Biosynthetic pathway for the production of (R)-prunasin in Prunus species (top) and Eucalyptus cladocalyx (bottom)
Biosynthesis of (R)-prunasin in Prunus dulcis
L-phenylalanine is first hydroxylated by CYP79D16, followed by a decarboxylation and dehydration, forming the E-oxime phenylacetaldoxime.[8] Next, CYP71AN24 catalyzes the rearrangement of the E-oxime to the Z-oxime followed by a dehydration and a hydroxylation to form mandelonitrile.[8] Finally, UGT85A19 or UGT94AF3 utilize UDP-glucose to glycosylate mandelonitrile, forming (R)-prunasin.[1]
After generating (R)-prunasin, the product is further glycosylated into amygdalin by either isoform UGT94AF1 or UGT94AF2.[1] Expression of UGTAF1/2 and prunasin hydrolases results in a low overall concentration of (R)-prunasin in almond tissues. It is important to note that an alpha-glucosidase or prunasin hydrolase can convert (R)-prunasin to mandelonitrile, its precursor, which can then be spontaneously or enzymatically hydrolyzed to benzaldehyde and hydrogen cyanide.[9]
Biosynthesis of (R)-prunasin in Eucalyptus cladocalyx
The biosynthesis of (R)-prunasin in E. cladocalyx, the sugar gum tree, has been shown to synthesize (R)-prunasin using an additional intermediate, phenylacetonitrile, using CYP706C55.[10] The pathway proceeds similarly to the pathway in Prunus species, where the multifunctional CYP79A125 catalyzes the conversion of L-phenylalanine to phenylacetaldoxime. Then, CYP706C55 catalyzes the dehydration of phenylacetaldoxime to phenylacetonitrile. Phenylacetonitrile is then hydroxylated by CYP71B103 to mandelonitrile. After generating mandelonitrile, UGT85A59 transfers glucose to yield (R)-prunasin.[10]
Metabolic Pathway Interactions
As (R)-prunasin is a product of secondary metabolism, its generation and degradation affect multiple metabolic pathways by consuming L-phenylalanine or increasing quantities of benzaldehyde and toxic hydrogen cyanide through prunasin degradation.
Metabolic profiling in almond, cassava, and sorghum identified a potential recycling mechanism where (R)-prunasin and other cyanogen glycosides may be utilized for nitrogen storage and nitrogen recycling without generating HCN.[11] In 2017, researchers used stable isotope labeling to demonstrate that 13C-labeled L-phenylalanine incorporated in (R)-prunasin could be converted to benzaldehyde and to salicylic acid using mandelonitrile as an intermediate.[12]
Toxicity
The toxicity of prunasin is based in its degradation products: (R)-prunasin is hydrolyzed to form benzaldehyde and hydrogen cyanide, which causes toxicity. Plants containing prunasin may therefore be toxic to animals, particularly ruminants.[13]
^Nahrstedt, Adolf; Rockenbach, Jürgen (1993). "Occurrence of the cyanogenic glucoside prunasin and II corresponding mandelic acid amide glucoside in Olinia species (oliniaceae)". Phytochemistry. 34 (2): 433. Bibcode:1993PChem..34..433N. doi:10.1016/0031-9422(93)80024-M.
^Andrew Pengelly (2004), The Constituents of Medicinal Plants (2nd ed.), Allen & Unwin, pp. 44–45, ISBN978-1-74114-052-1
