Two's company, three's a crowd: can H2S be the third endogenous gaseous transmitter?

Hydrogen sulfide-induced relaxation of resistance mesenteric artery beds of rats

Hydrogen sulfide (H2S) has been shown recently to function as an important gasotransmitter. The present study investigated the vascular effects of H2S, both exogenously applied and endogenously generated, on resistance mesenteric arteries of rats and the underlying mechanisms. Both H2S and NaHS evoked concentration-dependent relaxation of in vitro perfused rat mesenteric artery beds (MAB). The sensitivity of MAB to H2S (EC50, 25.2 +/- 3.6 microM) was about fivefold higher than that of rat aortic tissues. Removal of endothelium or coapplication of charybdotoxin and apamin to endothelium-intact MAB significantly reduced the vasorelaxation effects of H2S. The H2S-induced relaxation of MAB was partially mediated by ATP-sensitive K+ (KATP) channel activity in vascular smooth muscle cells. Pinacidil (EC50, 1.7 +/- 0.1 microM, n=6) mimicked, but glibenclamide (10 microM, n=6) suppressed, the vasorelaxant effect of H2S. KATP channel currents in isolated mesenteric artery smooth muscle cells were significantly augmented by H2S. L-cysteine, a substrate of cystathionine-gamma-lyase (CSE), at 1 mM increased endogenous H2S production by sixfold in rat mesenteric artery tissues and decreased contractility of MAB. DL-propargylglycine (a blocker of CSE) at 10 microM abolished L-cysteine-dependent increase in H2S production and relaxation of MAB. Our results demonstrated a tissue-specific relaxant response of resistance arteries to H2S. The stimulation of KATP channels in vascular smooth muscle cells and charybdotoxin/apamin-sensitive K+ channels in vascular endothelium by H2S represents important cellular mechanisms for H2S effect on MAB. Our study also demonstrated that endogenous CSE can generate sufficient H2S from exogenous L-cysteine to cause vasodilation. Future studies are merited to investigate direct contribution of endogenous H2S to regulation of vascular tone.

Polymorphisms of glutathione S-transferase M1 and T1 modulate blood pressure of individuals chronically exposed to natural sour gas containing sulfur compounds

In order to find the effect of genetic polymorphisms of GSTM1 and GSTT1 on blood pressure of individuals chronically exposed to sulfur compounds, the present study was done. Study subjects (38 males, 38 females) were residents of contaminated areas of Masjid-i-Sulaiman (southwest of Iran). The GSTM1 and GSTT1 genotypes were determined using a polymerase chain reaction (PCR)-based method. The non-parametric Sign test was applied in order to detect differences between the GSTs genotypes of study subjects and the normal mean values according to the sex and age of subjects. From four combination of genotypes, systolic blood pressure significantly decreased in combination of null-GSTM1 and present-GSTT1 (Z=-2.41; P=0.016), and diastolic blood pressure significantly increased in combination of present-GSTM1 and null-GSTT1 (Z=+2.14; P=0.032). It is speculated about polymorphisms of GSTs in individuals chronically exposed to natural sour gas, which contains H2S, fulfilling a physiological role(s) in regulating blood pressure.

Impact of hydrogen sulfide on carbon monoxide/heme oxygenase pathway in the pathogenesis of hypoxic pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) is an important pathophysiological process of a variety of cardiac and pulmonary diseases. But the mechanisms responsible for HPH are still not fully understood. The discoveries of endogenous gas signal molecules, nitric oxide (NO), and carbon monoxide (CO), have been moving the research of HPH to a new phase. Hydrogen sulfide (H2S), which is now being considered as the third new gas transmitter, was found to be possibly involved in the pathogenesis of HPH. But whether there exists an interaction between H2S and CO has not been clear in the pathogenesis of HPH. In this study, we found that H2S was significantly decreased in the pathogenesis of HPH. However, plasma CO level and the expressions of heme oxygenase (HO-1) protein and HO-1 mRNA were significantly increased. Exogenous supply of H2S could alleviate the elevation of pulmonary arterial pressure. At the same time, plasma CO level and the expressions of HO-1 protein and mRNA in pulmonary arteries were significantly increased. Whereas, exogenous supply of propargylglycine (PPG), an inhibitor of cystathionine gamma-lyase (CSE), decreased the plasma H2S content and worsened HPH. At the same time, plasma CO level and the expressions of HO-1 protein and mRNA in pulmonary arteries were decreased. The results showed that H2S could play a regulatory role in the pathogenesis of HPH through up-regulating CO/HO pathway.

Hydrogen sulphide: an endogenous stimulant of capsaicin-sensitive primary afferent neurons?

Hydrogen sulphide (H(2)S) is a gas best known for its rotten egg smell. The toxic effects of high concentrations of H(2)S have been extensively investigated. It is known that H(2)S is generated in mammalian systems, but little is known of its effects in physiological concentrations. In the present issue of this journal, Patacchini et al. present evidence that H(2)S stimulates capsaicin-sensitive primary afferent neurons to release tachykinins in the rat urinary bladder. The possible significance of this finding is discussed in this commentary.

Hydrogen sulfide as an endogenous regulator of vascular smooth muscle tone in trout

Hydrogen sulfide (H(2)S) is an endogenous vasodilator in mammals, but its presence and function in other vertebrates is unknown. We generated H(2)S from NaHS and examined the effects on isolated efferent branchial arteries from steelhead (stEBA) or rainbow (rtEBA) trout. H(2)S concentration was measured colorimetrically (CM) and with ion-selective electrodes (ISE) in rainbow trout plasma. NaHS produced a triphasic response consisting of a relaxation (phase 1), constriction (phase 2), and relaxation (phase 3) in both unstimulated vessels and in stEBA precontracted with carbachol (Carb). Phase 1 and phase 3 in stEBA were decreased and phase 2 increased in unstimulated vessels by K(+)(ATP) channel inhibition (glibenclamide), or a cocktail of inhibitors of cyclooxygenase, lipoxygenase, and cytochrome P-450 (indomethacin, esculetin, and clotrimazole). Inhibition of soluble guanylate cyclase with ODQ o NS-2028 inhibited phase 3 in stEBA, although NaHS decreased cGMP production by tEBA. stEBA phase 2 contractions were partially inhibited by the myosin light chain kinase inhibitor, ML-9, but unaffected by L-type calcium channel inhibition (methoxyverapamil), whereas contraction in tEBA was partially inhibited by nifedipine or removal of extracellular calcium. Phase 3 relaxations were more pronounced in stEBA precontracted with Carb and no epinephrine (NE) than those cont acted by KCl or K(2)SO(4). stEBA phase 2 and phase 3 responses were dose dependent (EC(50) = 1.1 +/- 1.2 x 10(-3) M and 6.7 +/- 0.9 x 10(-5) M, respectively; n = 7). NaHS was also vasoactive in steelhead bulbus arteriosus, celiac mesenteric arteries, and anterior cardinal veins. Rainbow trout plasma sulfide concentration was 4.0 +/- 0.3 x 10(-5) M, n = 4 (CM) and 3.8 +/- 0.4 x 10(-5) M, n = 9 (ISE); similar to phase 3 EC(50). Because NaHS has substantial vasoactive effects at physiological plasma concentrations, we propose that its soluble derivative, H(2)S, is a tonically active endogenous vasoregulator in trout.

Hydrogen sulfide: from the smell of the past to the mediator of the future?

Gases such as nitric oxide and carbon monoxide play important roles both in normal physiology and in disease. In recent years, interest has been directed towards other naturally occurring gases, notably hydrogen sulfide (H(2)S), which is both a potent vasodilator and a mediator of long-term potentiation in the brain. This article focuses on recent work that suggests a role for H(2)S, and perhaps other gases, in the CNS and cardiovascular system.

H2S generated by heart in rat and its effects on cardiac function

Hydrogen sulfide (H2S), which was considered as a novel gasotransmitter, is produced endogenously from L-cysteine in mammalian brain and vessels, and might be a physiological function regulator to these organs. Here, we showed that mRNA for H2S producing enzyme, cystathionine gamma-lyase, was expressed in myocardial tissues and H2S could endogenously be produced in myocardial tissues. Negative inotropic effect of H2S was proved in present study in vitro and in vivo experiments, and the effect could partly be blocked by glibenclamide, a KATP channel blocker. An intravenous bolus injection of NaHS provoked a decrease in central venous pressure. The present findings suggested that H2S could be endogenously produced by heart tissues, as a physiological cardiac function regulator, mediated by KATP channel pathway.

The possible role of hydrogen sulfide on the pathogenesis of spontaneous hypertension in rats

Hydrogen sulfide (H(2)S) is a newly found modulator in vascular system. This work showed that gene expression of cystathionine gamma-lyase (CSE), a H(2)S generating enzyme, and the activity of CSE in thoracic aorta were suppressed in hypertension rats. The plasma level of H(2)S also decreased in those rats. Exogenous administration of H(2)S could increase the plasma level of H(2)S and enhance the CSE activity of aorta. Exogenous administration of H(2)S also attenuated the elevation of pressure and lessened the aorta structural remodeling during the development of hypertension. In WKY rats, the gene expression and activity of CSE also decreased when the endogenous production of H(2)S was deprived by administration of DL-propargylglycine (specific inhibitor of CSE), accompanying the elevated pressure and the development of vascular remodeling. The results showed that endogenous H(2)S system was involved in both the maintenance of basal blood pressure and the development of hypertension. Exogenous H(2)S could exert beneficial effect on the pathogenesis of spontaneous hypertension.

Immunocytochemical evidence for nitric oxide- and carbon monoxide-producing neurons in the stomatogastric nervous system of the crayfishCherax quadricarinatus

Nitric oxide (NO) and carbon monoxide (CO) have been shown to serve neuromodulatory roles in both vertebrates and invertebrates. Here, we use antibodies to their respective biosynthetic enzymes, nitric oxide synthase (NOS) and heme oxygenase 2 (HO-2), to map the distribution of putative gas-producing neurons in the stomatogastric nervous system (STNS) of the crayfish Cherax quadricarinatus. In this species, NOS immunolabeling is found in the neuropil of the stomatogastric ganglion (STG). This staining originates from two immunopositive axons that project to the STG through the superior oesophageal and stomatogastric nerves, presumably from cell bodies located in the commissural ganglia (CoGs). HO-2 immunoreactivity is present in small diameter fibers and varicosities in the periphery of nerves located in the anterior portion of the STNS. This labeling originates from approximately 12 somata in each CoG. Transmission electron microscopy done on the nerves of the anterior STNS shows they contain a neuroendocrine plexus. Collectively, our results indicate that NO- and CO-producing neurons are likely to exist in the crayfish STNS. Moreover, these gases appear to be produced by distinct subsets of the neurons present there. The localization of NO to the STG neuropil suggests that it serves as a locally released modulator or is involved in the local release of other substances within this ganglion. The presence of CO in the neurohemal plexus of the anterior STNS suggests that it serves as a circulating hormone or is involved in the control of neuroendocrine release from this plexus.

The role of hydrogen sulfide generation in the pathogenesis of hypertension in rats induced by inhibition of nitric oxide synthase

OBJECTIVE

The present study intended to investigate whether the impaired H2S synthase/H2S pathway is associated with hypertension.

METHODS

Hypertension in Wistar rats was induced by the oral administration of the l-arginine analog, NG-nitro-l-arginine methyl ester (l-NAME) in their drinking water for a period of 6 weeks. The control rats were given plain tap water only. Sodium hydrosulfide (NaHS) was given by intraperitoneal injection to both the control group and the l-NAME-treated group. The systolic BP (blood pressure) was measured by a tail-cuff method using a pulse transducer. Plasma hydrogen sulfide (H2S), and H2S generation by thoracic aorta and superior mesenteric artery, were determined. In addition, the activity of cystathionine-gamma-lyase (CSE) in thoracic aorta and superior mesenteric artery, most responsible for H2S production, was also measured. Competitive reverse transcriptase-polymerase chain reaction (RT-PCR) was used to determine CSE mRNA in thoracic aorta.

RESULTS

l-NAME caused a time-dependent elevation of systolic BP. The heart-to-body weight ratio of l-NAME-treated rats was 27% higher than that of controls. The systolic BP in the NaHS-treated l-NAME group was significantly decreased, by 19% (P < 0.01), in comparison with the l-NAME group. The heart-to-body weight ratio decreased significantly by 12%. l-NAME inhibited CSE gene expression significantly. The inhibition of H2S generation and CSE activity by l-NAME was greatly attenuated in the NaHS-treated l-NAME group. However, there was no significant difference in nitric oxide (NO) generation between the l-NAME group and the NaHS-treated l-NAME group.

CONCLUSION

In summary, dysfunction of the vascular H2S synthase/H2S pathway was found in l-NAME-induced hypertensive rats. Exogenous H2S effectively prevented the development of hypertension induced by l-NAME. These findings suggest that the H2S synthase/H2S pathway participates in hypertension.

The gasotransmitter role of hydrogen sulfide

A novel concept of "gasotransmitter" arrived recently. Gasotransmitters are small molecules of endogenous gases with important physiological functions. Their production and metabolism are enzymatically regulated, and their effects are not dependent on specific membrane receptors. Following the identification of nitric oxide and carbon monoxide as gasotransmitters, hydrogen sulfide (H(2)S) may be qualified as the third gasotransmitter. Recent studies have shown that H(2)S is generated from vascular smooth muscle cells (SMCs), catalyzed by specific H(2)S-generating enzyme. At physiologically relevant concentrations, H(2)S relaxes vascular tissues, an effect mediated by the activation of ATP-sensitive K(+) (K(ATP)) channels in vascular SMCs. H(2)S directly alters the activity of K(ATP) channels without the involvement of second messengers. Furthermore, the endogenous production of H(2)S in the cardiovascular system is likely regulated by nitric oxide, whereas the vasorelaxant effect of nitric oxide is inhibited by H(2)S. It is anticipated that future studies will better reveal the molecular mechanisms underlying the effect of H(2)S on K(ATP) channel proteins, the interaction of H(2)S and other gasotransmitters in cardiovascular system, the endogenous stimulators and inhibitors of H(2)S metabolism, the role of H(2)S in the regulation of heart function, and the abnormal H(2)S production and action under various pathophysiological conditions.

Chronic carbon monoxide exposure of hypoxic rats increases in vitro sensitivity of pulmonary artery smooth muscle

Exogenous carbon monoxide (CO) induces pulmonary vasodilation by acting directly on pulmonary artery (PA) smooth muscle cells. We investigated the contribution of K+ channels and soluble guanylyl cyclase to the regulation of PA tone by acute CO in chronic hypoxic rats (3 weeks at 0.5 atm (1 atm = 101.325 kPa); hypoxic) and in chronic hypoxic rats exposed to exogenous CO (3 weeks at 0.5 atm + 50 ppm CO; hypoxic-CO). Acute CO induced relaxation in PA rings from all animals. However, the amplitude of CO relaxation was significantly decreased in hypoxic rings and increased in hypoxic-CO rings. This different effect occurred with a decrease and an increase of pD2, respectively, in hypoxic and hypoxic-CO rings. We showed a positive relation between the percentage of inhibition of CO relaxation by a blocker of K+ channels and the increase of CO sensitivity. Thus, we showed for the first time that chronic hypoxia decreases acute CO sensitivity, which in contrast, increases in the presence of chronic CO. The present study provides initial evidence of a link between increased K(+)-channel activity and CO sensitivity.

Age-related alterations in soluble guanylyl cyclase and cGMP pathway in spontaneously hypertensive rats

BACKGROUND

Vascular contractility and blood pressure (BP) are regulated by soluble guanylyl cyclase (sGC) and cyclic guanosine monophosphate (cGMP) pathway, which can be influenced by heme oxygenase (HO)-derived carbon monoxide (CO). The age-related changes in sGC/cGMP pathway in tail artery smooth muscle cells (SMCs) in hypertension have not been systematically investigated.

METHODS

In the present study, spontaneously hypertensive rats (SHR) of 4, 8, and 20 weeks old were used. The basal and hemin-modulated levels of sGC and cGMP in tail artery tissues were examined.

RESULTS

Although BP of 20-week SHR was significantly elevated, sGC and cGMP levels were unaltered compared with age-matched Wistar-Kyoto rats (WKY). The levels of sGC and cGMP were significantly lower in 4- and 8-week SHR compared with age-matched WKY although BP of 4-week SHR was normotensive. Hemin administration resulted in a significant decrease in BP in 8-week (158.7 +/- 2.4 versus 123.5 +/- 1.3 mmHg, P < 0.01), but not in pre-hypertensive (4 weeks) or 20-week SHR or WKY at all ages. Coincidently, sGC and cGMP levels in 8-week SHRs were significantly elevated and so did the expression levels of HO-1. Hemin treatment did not increase the cyclic adenosine monophosphate (cAMP) content of tail artery from 8-week SHR. The constitutive HO-2 levels remained unchanged in 8- and 20-week SHR and age-matched WKY.

CONCLUSION

The HO-activity inhibitor, chromium mesoporphyrin, abolished the BP-lowering and HO- stimulating effects of hemin in young SHR. Our results suggest that alteration in sGC/cGMP pathway in vascular SMCs precedes the occurrence of hypertension but returns to normal once hypertension is fully manifested.

Induction of heme oxygenase-1 and stimulation of cGMP production by hemin in aortic tissues from hypertensive rats

Heme oxygenase (HO) and carbon monoxide (CO) have been implicated in the modulation of various cardiovascular functions including blood pressure (BP) regulation. Up-regulating the HO/CO system lowers BP in young (8-week-old) but not in adult (20-week-old) spontaneously hypertensive rats (SHRs). The mechanisms for this selective effect are largely unknown. We investigated the effects of HO-1 inducer, hemin, on the HO/CO-soluble gyanylyl cyclase (sGC)/cGMP system in the aorta of prehypertensive (4-week-old) young and adult SHRs as well as age-matched Wistar-Kyoto rats (WKYs). Reduced expressions of HO-1, HO-2, and sGC proteins associated with depressed HO activity and cGMP levels were detected in young SHRs. These deficiencies were significantly reversed by hemin treatment. Macrophage infiltration of vascular tissues was more significant in adult SHRs than adult WKYs, but invisible in young SHRs and WKYs. Hemin treatment did not alter macrophage infiltration of vascular tissues in young SHRs. The same hemin administration resulted in a significant decrease in BP (from 148.6 +/- 3.2 to 125.8 +/- 2.6 mmHg, P <.01) in young SHRs, but not in prehypertensive or adult SHRs or WKYs of all ages. The HO inhibitor zinc protoporphyrin abrogated the hemin effect in young SHRs. Aortic tissues became desensitized to YC-1, an activator sGC, in adult SHRs. Thus, in young SHRs the expression and function of the HO/CO-sGC/cGMP system were suppressed, constituting a pathogenic mechanism for the development of hypertension. In adult SHRs, the HO/CO-sGC/cGMP system appeared normal, but desensitization of the sGC/cGMP pathway caused hypertension to prevail.

Tomatoes, lycopene and prostate cancer: a clinician's guide for counseling those at risk for prostate cancer

Prostate cancer has become a major public health issue and the search for etiologic risk factors and the development of chemopreventive agents has gained momentum over the last decade. An important epidemiologic finding has been the association between the consumption of tomato products and a lower risk of prostate cancer. Several investigators have proposed that lycopene, a carotenoid consumed largely from tomato products, may be the component responsible for lowering the risk of prostate cancer. Laboratory and clinical studies have been initiated with the goal of assessing the ability of pure lycopene to serve as a chemopreventive agent for prostate cancer. The focus on lycopene should continue, and an improved understanding of lycopene absorption, distribution, role in antioxidant reactions, and metabolism is critical in the quest to elucidate mechanisms whereby this compound may possibly reduce prostate cancer risk.

Alterations in heme oxygenase/carbon monoxide system in pulmonary arteries in hypertension

Enhancement of the heme oxygenase/carbon monoxide (HO/CO) system has been shown to lower blood pressure (BP) in young (8 weeks), but not in adult (20 weeks) spontaneously hypertensive (SHR) rats. The reasons for this selective effect still remain puzzling. We investigated the effects of hemin on the HO/CO system of the pulmonary artery (PA) in SHR and Wistar-Kyoto (WKY) rats at different ages and evaluated the hemin-dependent changes in sGC and cGMP pathways. Hemin administration resulted in an evident reduction of BP (from 148.6 +/- 3.2 to 125.8 +/- 2.6 mmHg, P < 0.01) in young, but not in prehypertensive (4 weeks) or adult SHR or WKY rats at all ages. Coadministration of the HO inhibitor, chromium mesoporphyrin, with hemin, cancelled the BP-lowering effect of hemin. Remarkably, lower expression levels of HO-1, HO-2, and sGC paralleled with reduced HO activity and cGMP content were observed in PA from 8-week SHR rats, but not from adult SHR or WKY rats of all ages. Interestingly, hemin treatment restored these deficiencies, although the expression level of non-inducible HO-2 protein remained unchanged. We conclude that in young and prehypertensive SHR rats, an impaired HO/CO-sGC/cGMP system in the PA might be indicative of the pathogenesis and development of hypertension. In contrast, the HO/CO system in the PA of adult SHR rats was upregulated as a compensatory reaction to elevated BP and desensitization of the downstream targets of the sGC/cGMP pathway occurred.

Interaction of selective amino acid residues of K(ca) channels with carbon monoxide

The activation of big-conductance K(Ca) channels in vascular smooth muscle cells by carbon monoxide (CO) has been demonstrated previously. One specific target of CO on K(Ca) channel proteins is the histidine residue. The roles of other amino acid residues on the functionality of K(Ca) channels, as well as their reactions to CO, have been unclear. In the present study, the cell-free single channel recording technique was used to investigate the chemical modification of K(Ca) channels by CO and other chemical agents. The modification of negatively charged carboxyl groups and the epsilon -amino group of lysine did not affect the open probability, but decreased single-channel conductance of K(Ca) channels. When sulfhydryl groups of cysteine were modified with N-ethylmaleimide, the open probability of K(Ca) channels was decreased, but single-channel conductance was not affected. None of the above chemical modifications affected the CO-induced increase in the open probability of K(Ca) channels. However, N-ethylmaleimide treatment reduced the stimulatory effect of nitric oxide (NO) on K(Ca) channels. Finally, pretreatment of smooth muscle cells with NO abolished the effects of subsequently applied CO on K(Ca) channel proteins. Our study demonstrates that CO and NO acted on different amino acid residues of K(Ca) channel proteins. The interaction of CO and NO determines the functional status of K(Ca) channels in vascular smooth muscle cells

The regulatory effect of hydrogen sulfide on hypoxic pulmonary hypertension in rats

Hypoxic pulmonary hypertension (HPH) is an important pathophysiological process. The mechanism of HPH is still not fully understood. Recent studies showed that hydrogen sulfide (H(2)S) could relax vascular smooth muscles and inhibit the proliferation of cultured vascular smooth muscle cells. Our study showed that both the gene expression of cystathionine gamma-lyase (CSE), one of the H(2)S generating enzymes, and the activity of CSE were suppressed in lung tissues during HPH. And the plasma level of H(2)S was decreased during HPH. Exogenous supply of H(2)S could increase the plasma level of H(2)S, enhance CSE activity, and up-regulate CSE gene expression in lung tissue. At the same time, exogenous supply of H(2)S could oppose the elevation of pulmonary arterial pressure and lessen the pulmonary vascular structure remodeling during HPH. The results showed that endogenous H(2)S system was involved and exogenous H(2)S could exert beneficial effect on the pathogenesis of HPH.

Inhibition of carcinogenesis by minor anutrient constituents of the diet

A continuing study of chemopreventive agents has focused on several categories of naturally occurring compounds that inhibit carcinogen activation and are effective in preventing carcinogen-induced neoplasia when administered at short time-intervals before carcinogen challenge. The inhibitory compounds are: aromatic isothiocyanates found in cruciferous vegetables, monoterpenes present in citrus fruits and caraway-seed oil, and organosulphur compounds occurring in Allium species. Preliminary work indicates that glucobrassicin and indoles existing in cruciferous vegetables also have these attributes. Almost all carcinogens that are consumed in food require metabolic activation. Thus, inhibition of carcinogen activation reactions could be effective against this type of exposure. In addition, three naturally occurring compounds, i.e. phenethyl isothiocyanate, D-limonene and dipropyl sulphide inhibit activation of the tobacco-specific carcinogen NNK, and accordingly may have the capacity to diminish carcinogenic response to exposures to tobacco. The property of cruciferous vegetables, orange oil, benzyl isothiocyanate, and D-limonene, to act as both blocking and suppressing agents has been discussed. Two possible mechanisms for this multi-phase activity were presented. The first is that these inhibitory substances activate a complex integrated defence mechanism against toxic compounds which entails both blocking and suppressing components. The blocking component is the initial line of defence, and the suppressing component constitutes a 'fail-safe' backup to assure that if any of the toxic material attacks cellular constituents, its effects will be nullified. The second possible mechanism considered is that the inhibitors, because of high reactivity, have multiple biological effects that are separate and not part of a single, coordinated response. Inhibitors that have both blocking and suppressing effects could be particularly useful as chemopreventive agents. A simple interim classification of foods in terms of their potential impact on the occurrences of cancer has been proposed.