Polished rice as natural sources of cancer-preventing geranylgeranoic acid

Abstract

Geranylgeranoic acid, a 20-carbon polyprenoic acid (all-trans 3,7,11,15-tetramethyl-2,4,6,10,14-hexadecatetraenoic acid) and its derivatives were previously developed as synthetic "acyclic retinoids" for cancer chemoprevention. Recently, we demonstrated the natural occurrence of geranylgeranoic acid in various medicinal herbs (Shidoji and Ogawa, 2004). In this present study, we present several lines of evidence to demonstrate that geranylgeranyl diphosphate taken in foods could be metabolized to GGA through geranylgeraniol and geranylgeranyl aldehyde via the following steps: 1) The conversion from geranylgeranyl diphosphate to geranylgeraniol was demonstrated to occur by the action of bovine intestinal alkaline phosphatase, with a K(m) of 46.1 µM. 2) Geranylgeraniol oxidase-mediated conversion of geranylgeraniol to geranylgeranyl aldehyde was revealed in rat liver homogenates, which activity was mainly localized in the mitochondrial fraction. The mitochondrial enzyme showed a K(m) of 92.9 µM. 3) The conversion of geranylgeranyl aldehyde to geranylgeranoic acid by geranylgeranyl aldehyde dehydrogenase in rat liver homogenates was absolutely dependent on exogenously added NAD(+) or NADP(+). The K(m) of the mitochondrial geranylgeranyl aldehyde dehydrogenase was 27.5 µM for geranylgeranyl aldehyde. Taken together, our data suggest that cancer preventive geranylgeranoic acid could be a physiological metabolite from commonly consumed foods.

Keywords: cancer chemoprevention; geranylgeraniol; geranylgeranoic acid; geranylgeranyl diphosphate; polished rice.

Fig. 1

Schematic diagram of the isoprenoid production. Pathways via acetate/mevalonate (MVA) or GAP/pyruvate [methylerythritol phosphate (MEP)] are shown. The details are described in the text.

Fig. 2

Dephosphorylation of geranylgeranyl diphosphate (GGPP). Reversed-phase HPLC chromatogram of the lipid extracts from Kinuhikari polished rice before (a) and after (b) KOH treatment, and from 100 µM GGPP solution in 50 mM Tris-HCl buffer (pH 9.8) without (c) or with (d) bovine ALP. Details of the experimental procedure were described in Materials and Methods. Arrows indicate the elution position of authentic GGOH.

Fig. 3

Enzymatic conversion of geranylgeraniol (GGOH) to geranylgeranoic acid (GGA) via geranylgeraniol (GGal) in rat liver. Shown are HPLC profiles of the diazomethane-treated ether extracts from the reaction mixtures of rat liver homogenates (0.5 mg protein) with or without 200 µM GGOH (a), with 200 µM GGOH and 5 mM NAD⁺ (b) and with 200 µM GGal in the absence or presence of NAD⁺ (c). The elution profiles of diazomethane-treated authentic GGal and GGA are shown in the lowest chromatogram in panels a and c, respectively.

Fig. 4

Proposed enzymatic conversion from geranylgeranyl diphosphate (GGPP) to geranylgeranoic acid (GGA).