*6.1.4. Vitamins A, C and E*

Vitamin A plays a role in the development of both T-helper cells and B-cells. Studies have shown that vitamin A, in the form of retinol or retinoic acid, improves immunity by stimulating immunoglobulin synthesis through its action on T-cells or T-cell products. Retinoic acid inhibits the production of interleukin-6 in a dose-dependent manner by downregulating the expression of interleukin-6 mRNA [84]. Vitamin A acts as an immunostimulant by modulating the growth and function of T-cells, B-lymphocytes and natural killer cells.

Vitamin C (ascorbate, AscH- ) can donate a hydrogen atom to a free radical molecule (R<sup>⋅</sup> ), thereby neutralising it while becoming an ascorbate radical itself (Asc or Asc⋅-). But the free radical (Asc<sup>⋅</sup> ) is very stable because of its resonance structure. Moreover, AscH is readily regenerated from the Asc- with NADH or NADPH-dependent reductases [85]. Ascorbate can also neutralise the radical form of other antioxidants such as glutathione (GS<sup>⋅</sup> ) and vitamin E (<sup>⋅</sup> TOC). Vitamin C also inhibits the replication of human immunodefi‐ ciency virus-1 (HIV-1) [86].

Tocopherols (vitamin E) interrupt free radical chain reactions by capturing the free radical; this inherent action displays the antioxidant properties of vitamin E. The free hydroxyl (OH) group on the aromatic ring of tocopherol is responsible for its antioxidant activity. The hydrogen from this group is transferred to the free radical, resulting in a relatively stable free radical form of the vitamin [87]. Vitamin E is an effective antioxidant (peroxyl radical scav‐ enger) for terminating the chain reactions of lipid peroxidation in the cell membrane. The tocopheroxyl radical is the pro-oxidant form of vitamin E and is thought to be regenerated to the antioxidant form by a network of other antioxidants, including vitamin C and glutathione. In the mitochondria membrane, vitamin E that donates a hydrogen to neutralise a free radical can be regenerated (reduced) by coenzyme Q which has two hydrogens to donate and can avoid becoming a free radical by donating both hydrogens; this is an efficient process. Alphatocopherol has potent activity against HIV. The anti-HIV-1 activity may be due, in part, to their antioxidant properties. Alpha-tocopherol generally interferes with membrane integrity and fluidity. As HIV-1 is a membrane virus, any alteration of the membrane fluidity of the virus interferes with its ability to bind to cell-receptor sites, thus reducing its infectivity [88]. It stimulates CD4 T-cell and IL-2 proliferation [89]. Vitamin E inhibits CD95 (APO–1/Fas) ligand expression (part of TNF receptor which T-cell uses to undergo apoptosis) and protects T-cell from activation-induced cell death of the CD95/CD95 ligand system of T-cells [90]. Tocopherol completely inhibits and blocks DNA binding NF-κB, resulting to complete inaction [91].

#### *6.1.5. Flavonoids*

The flavones and catechins seem to be the most powerful flavonoids for protecting the body against ROS [92]. Flavonoids may have an additive effect to the endogenous scavenging compounds; they increase their function. Flavonoids (quercetin) was reported to exhibit both anti-infective and antireplicative HIV abilities. Quercetin significantly downregulates p24 antigen production, LTR gene expression and viral infectivity in a dose-dependent manner (5– 50 mM) and further downregulation of the expression of the pro-inflammatory cytokine TNFα with concomitant upregulation of anti-inflammatory cytokine IL-13 [93]. A higher level of IL-13 is known to inhibit TNF-α production and also HIV-1 infection. Some flavonoids work on the intracellular replication of viruses, whereas others inhibit the infectious properties of the viruses. Flavonoids have inhibitory activity on reverse RT and RNA-directed DNA polymerase [94, 95]; however, it also has antiintegrase and antiprotease activities [96]. Similarly, myricetin activity was tested against HIV-RT and inhibited the enzyme by 49 % [97]

#### *6.1.6. Metals*

Zinc is a metallic divalent cation bound to proteins within cells and cell membranes. Zinc plays catalytic, structural and regulatory roles in more than 200 zinc metalloenzymes that have been identified in biological systems. Zinc fingers are exploited by transcription factors for inter‐ acting with DNA and regulating the activity of genes [98]. Another structural role of zinc is in the maintenance of the integrity of biological membranes (membrane stabilisation) by its ability to stabilise thiol groups and phospholipids, resulting in their protection against oxidative injury. These properties affect signalling processes involved in cell-mediated immunity. Zinc also influences gene expression by structural stabilisation of different immu‐ nological transcription factors. It induces cytokines, including interleukin (IL)-1, IL-6 and TNFα [99]. HIV binds to zinc ions in T-cells in order to produce proviral peptides, which form the basis of new infectious viral particles. HIV-1 protease enzyme cuts the viral chains to form new infectious viral particles, as with other proteases (collagenase, angiotensin-converting enzyme (ACE), caboxypeptidase A and neutral endopeptidases); when sufficient zinc ions are bound to the protease, it will remain inactive [100]. Zinc therefore has both an enhancing and inhibiting activity depending on its concentration in the surrounding tissues. In HIV replica‐ tion, viral RNA is transformed into viral DNA via the enzyme reverse transcriptase; zinc also binds to this enzyme. Zinc influences NK cell-mediated killing and also modulates cytolytic T-cell activity and inhibition of TNF-α [101], in addition to its anti-HIV drug potentiation activity as [102].

readily regenerated from the Asc- with NADH or NADPH-dependent reductases [85]. Ascorbate can also neutralise the radical form of other antioxidants such as glutathione

192 Trends in Basic and Therapeutic Options in HIV Infection - Towards a Functional Cure

Tocopherols (vitamin E) interrupt free radical chain reactions by capturing the free radical; this inherent action displays the antioxidant properties of vitamin E. The free hydroxyl (OH) group on the aromatic ring of tocopherol is responsible for its antioxidant activity. The hydrogen from this group is transferred to the free radical, resulting in a relatively stable free radical form of the vitamin [87]. Vitamin E is an effective antioxidant (peroxyl radical scav‐ enger) for terminating the chain reactions of lipid peroxidation in the cell membrane. The tocopheroxyl radical is the pro-oxidant form of vitamin E and is thought to be regenerated to the antioxidant form by a network of other antioxidants, including vitamin C and glutathione. In the mitochondria membrane, vitamin E that donates a hydrogen to neutralise a free radical can be regenerated (reduced) by coenzyme Q which has two hydrogens to donate and can avoid becoming a free radical by donating both hydrogens; this is an efficient process. Alphatocopherol has potent activity against HIV. The anti-HIV-1 activity may be due, in part, to their antioxidant properties. Alpha-tocopherol generally interferes with membrane integrity and fluidity. As HIV-1 is a membrane virus, any alteration of the membrane fluidity of the virus interferes with its ability to bind to cell-receptor sites, thus reducing its infectivity [88]. It stimulates CD4 T-cell and IL-2 proliferation [89]. Vitamin E inhibits CD95 (APO–1/Fas) ligand expression (part of TNF receptor which T-cell uses to undergo apoptosis) and protects T-cell from activation-induced cell death of the CD95/CD95 ligand system of T-cells [90]. Tocopherol completely inhibits and blocks DNA binding NF-κB, resulting to complete inaction [91].

The flavones and catechins seem to be the most powerful flavonoids for protecting the body against ROS [92]. Flavonoids may have an additive effect to the endogenous scavenging compounds; they increase their function. Flavonoids (quercetin) was reported to exhibit both anti-infective and antireplicative HIV abilities. Quercetin significantly downregulates p24 antigen production, LTR gene expression and viral infectivity in a dose-dependent manner (5– 50 mM) and further downregulation of the expression of the pro-inflammatory cytokine TNFα with concomitant upregulation of anti-inflammatory cytokine IL-13 [93]. A higher level of IL-13 is known to inhibit TNF-α production and also HIV-1 infection. Some flavonoids work on the intracellular replication of viruses, whereas others inhibit the infectious properties of the viruses. Flavonoids have inhibitory activity on reverse RT and RNA-directed DNA polymerase [94, 95]; however, it also has antiintegrase and antiprotease activities [96]. Similarly, myricetin activity was tested against HIV-RT and inhibited the enzyme by 49 % [97]

Zinc is a metallic divalent cation bound to proteins within cells and cell membranes. Zinc plays catalytic, structural and regulatory roles in more than 200 zinc metalloenzymes that have been identified in biological systems. Zinc fingers are exploited by transcription factors for inter‐

TOC). Vitamin C also inhibits the replication of human immunodefi‐

(GS<sup>⋅</sup>

) and vitamin E (<sup>⋅</sup>

ciency virus-1 (HIV-1) [86].

*6.1.5. Flavonoids*

*6.1.6. Metals*

Similarly, selenium is found in human and animal tissue as L-selenomethionine or L-seleno‐ cysteine. L-selenomethionine is incorporated randomly in proteins known as selenoproteins. The antioxidant activity of selenium is mainly accounted for by virtue of its role in the formation and function of the selenium-dependent glutathione peroxidase (GSHPx) [103]. Selenium effect on boosting cellular immunity is due to the upregulation of the expression of the lymphocyte cells' high affinity to interleukin (IL)-2 receptors, thus providing a vehicle for enhanced lymphocyte cell response as well as preventing oxidative stress to human cells [104,105]. Research has shown that the HIV virus hijacks the host supply of selenium for its own antioxidant protection, thereby inducing or exacerbating a selenium deficiency with increasing disease progression. Thus, HIV may be capable of incorporating host selenium into viral selenoprotein that has glutathione peroxidase activity [106,107].
