Oxidative Tryptophan Modification by Terpene- and Squalene-Hydroperoxides and a Possible Link to Cross-Reactions in Diagnostic Tests

Revisiting carotenoids as dietary antioxidants for human health and disease prevention

Humans are unique indiscriminate carotenoid accumulators, so the human body accumulates a wide range of dietary carotenoids of different types and to varying concentrations. Carotenoids were once recognized as physiological antioxidants because of their ability to quench singlet molecular oxygen (1O2). In the 1990s, large-scale intervention studies failed to demonstrate that supplementary β-carotene intake reduces the incidence of lung cancer, although its antioxidant activity was supposed to contribute to the prevention of oxidative stress-induced carcinogenesis. Nevertheless, the antioxidant activity of carotenoids has attracted renewed attention as the pathophysiological role of 1O2 has emerged, and as the ability of dietary carotenoids to induce antioxidant enzymes has been revealed. This review focuses on six major carotenoids from fruit and vegetables and revisits their physiological functions as biological antioxidants from the standpoint of health promotion and disease prevention. β-Carotene 9',10'-oxygenase-derived oxidative metabolites trigger increases in the activities of antioxidant enzymes. Lutein and zeaxanthin selectively accumulate in human macular cells to protect against light-induced macular impairment by acting as antioxidants. Lycopene accumulates exclusively and to high concentrations in the testis, where its antioxidant activity may help to eliminate oxidative damage. Dietary carotenoids appear to exert their antioxidant activity in photo-irradiated skin after their persistent deposition in the skin. An acceptable level of dietary carotenoids for disease prevention should be established because they can have deleterious effects as prooxidants if they accumulate to excess levels. Finally, it is expected that the reason why humans are indiscriminate carotenoid accumulators will be understood soon.

One hundred years of allergic contact dermatitis due to oxidized terpenes: What we can learn from old research on turpentine allergy

Although in recent years the focus on sensitizing terpene oxidation products has been on oxidized limonene and linalool, the autoxidation of terpenes in relation to allergic contact dermatitis is not new and dates back to the early part of the 20th century with the use of turpentine causing occupational contact dermatitis in painters. This review is written in a way as to allow us to get closer to the work of the scientists in earlier days, to participate in the successes, and also to observe the weak points. The researchers concluded that the main culprit in Scandinavian turpentine was Δ³ -carene hydroperoxides. This explains its high sensitizing effect compared with French turpentine which is of the Iberian type with no or only traces of Δ³ -carene. Historical exposure to turpentine showed that ending the industrial exposure stopped the occupational skin sensitization. Patch test studies demonstrated that monoterpene hydroperoxides, far from being an obsolete source of contact allergy solely related to turpentine, is a common cause of contact allergy in the population. A hundred years of extensive chemical and clinical studies worldwide should be sufficient to meet the evidence requirement regarding allergic contact dermatitis caused by terpenes. HIGHLIGHTS: The autoxidation of terpenes in relation to allergic contact dermatitis is not new and dates back to the early part of the 20th century with the use of turpentine. The main culprit in Scandinavian turpentine was Δ³ -carene hydroperoxides. This explains its high sensitizing effect compared with French turpentine with no or only traces of Δ³ -carene. Recent patch test studies demonstrated that monoterpene hydroperoxides, far from being an obsolete source of contact allergy solely related to turpentine, is a common cause of contact allergy in the population.

Current Insights into the Biological Action of Squalene

Squalene is a triterpenic compound found in a large number of plants and other sources with a long tradition of research since it was first reported in 1926. Herein a systematic review of studies concerning squalene published in the last 8 years is presented. These studies have provided further support for its antioxidant, anti-inflammatory, and anti-atherosclerotic properties in vivo and in vitro. Moreover, an antineoplastic effect in nutrigenetic-type treatments, which depends on the failing metabolic pathway of tumors, has also been reported. The bioavailability of squalene in cell cultures, animal models, and in humans has been well established, and further progress has been made in regard to the intracellular transport of this lipophilic molecule. Squalene accumulates in the liver and decreases hepatic cholesterol and triglycerides, with these actions being exerted via a complex network of changes in gene expression at both transcriptional and post-transcriptional levels. Its presence in different biological fluids has also been studied. The combination of squalene with other bioactive compounds has been shown to enhance its pleiotropic properties and might lead to the formulation of functional foods and nutraceuticals to control oxidative stress and, therefore, numerous age-related diseases in human and veterinary medicine.

Can contact allergy to p-phenylenediamine explain the high rates of terpene hydroperoxide allergy? - An epidemiological study based on consecutive patch test results

BACKGROUND

Contact allergy to linalool hydroperoxides (Lin-OOHs) and limonene hydroperoxides (Lim-OOHs) is common. Similarly to what occurs with the terpene hydroperoxides, reactive intermediates formed from p-phenylenediamine (PPD) can cause oxidative modifications of tryptophan residues on proteins in mechanistic studies.

OBJECTIVES

To test the hypothesis that patients sensitized to PPD are at increased risk of concomitant reactivity to either of the terpene hydroperoxides, owing to a 'common pathway' of skin protein oxidation.

METHODS

A database study of consecutively patch tested eczema patients (n = 3843) from 2012 to 2015, tested concomitantly with PPD, Lim-OOHs and Lin-OOHs, was performed. Associations were examined by level of concordance and odds ratios (ORs) adjusted for age, sex, and contact allergy to fragrance mix I and fragrance mix II.

RESULTS

Concomitant reactions to PPD were seen in 2.2% of Lim-OOH-positive patients and in 4.9% of Lin-OOH-positive patients. Neither proportion was higher than expected by chance. No association existed between PPD and Lim-OOH patch test reactivity. In a multiple logistic regression analysis, PPD allergy was associated with an insignificantly increased risk (OR 2.11, 95%CI:0.92-4.80) of a positive patch test reaction to Lin-OOHs.

CONCLUSIONS

PPD sensitization cannot explain the high rates of sensitization to Lin-OOHs and/or Lim-OOHs. Contact allergy to oxidized linalool is more strongly associated with fragrance allergy than with PPD allergy.