In vitro, cinnamic amides in tribulus fruits inhibit papain-like protease (PLpro), an essential proteolytic enzyme which plays a critical role in virus-mediated RNA replication ⁽⁴²⁾. Di-p-coumaroylquinic acid derivatives in tribulus are associated with its antioxidant effects ⁽⁴³⁾.

Aphrodisiac effects may be associated with the constituent protodioscin, which leads to an increase in some sex hormones ⁽⁴⁴⁾. Erectogenic effects may also occur via conversion of protodioscin to DHEA ⁽³⁶⁾ or by concentration-dependent relaxation of the corpus cavernosum (CC) via reactions in the nitric oxide (NO)/NO synthase pathway and CC endothelium ^(45) (25). The NO release effect may explain observed physiological responses to tribulus supplementation, independent of testosterone level ⁽²⁵⁾. In a study conducted in rats with ovarian cysts, tribulus extract showed a luteinizing effect related to gonadotropin-like activity ⁽²⁰⁾.

Saponins exhibit cytotoxic and antihyperlipidemic effects ⁽⁴²⁾. They may protect against atherosclerosis by suppressing angiotensin II-induced vascular smooth muscle cell proliferation via inhibition of intracellular ROS production, calcium ion mobilization, pkc-α expression, ERK1/2 phosphorylation, and proto-oncogene expression ⁽⁴⁶⁾. In ischemic cells, polyphenol-mediated antioxidant activity with tribulus extract resulted in significant suppression of LDH release, ROS generation, and superoxide production ⁽²⁹⁾.

In animal models, tribulus exerts a protective effect in diabetic rats by inhibiting oxidative stress ⁽⁶⁾ and lowering levels of glycosylated hemoglobin and cholesterols ⁽⁸⁾. In diabetic neuropathic pain models, tribulus modulates oxidative stress and inflammatory cytokine release in a dose-dependent manner via increases in superoxide dismutase, catalase, glutathione peroxidase, and reduced glutathione and lipid peroxide levels ⁽³²⁾. In chronic mild stress models, the attenuation of serum corticotropin-releasing factor and cortisol levels by tribulus saponins suggest normalization of hypothalamic-pituitary-adrenal (HPA) axis hyperactivity ⁽³¹⁾. Antihypertensive effects occur via arterial smooth muscle relaxation, NO release, and membrane hyperpolarization ⁽³⁾. An aqueous extract also demonstrated angiotensin converting enzyme (ACE)-inhibition activity ⁽²⁾ that may help lower blood pressure. Phenolic acids such as chlorogenic acid, caffeic acid and 4-hydroxybenzoic acid may be responsible for reputed cardioprotective properties ⁽²⁹⁾.

Tribulus extracts induce apoptosis and suppress cancer cell proliferation by activating caspase 3, dephosphorylating extracellular signal-related kinase (ERK) 1 and 2 ⁽¹⁵⁾, and by inhibiting nuclear factor (NF-kappa B) signaling ⁽¹²⁾. Saponins from tribulus inhibit multiple-drug resistance of cancer cells ⁽¹¹⁾. In breast carcinoma cell lines, a saponin extract changed mRNA levels of CXCR4, CCR7 and BCL2 genes ⁽³⁸⁾. In models of human prostate cancer, antitumor and antiangiogenic activities are attributed to cell-cycle arrest and apoptotic induction not involving the caspase pathway ⁽³⁹⁾. Saponins also act as a modulator of apoptosis: in normal human keratinocytes, saponins attenuate UVB-induced programmed cell death through inhibition of intrinsic apoptotic pathway, but enhance apoptotic response in squamous cell carcinomas ⁽⁴⁰⁾. The photoprotective effect of saponins is attributed to enhanced NER gene expression and inhibition of UVB-mediated NF-kappaB activation ⁽⁴⁰⁾. In animal models, a hydroalcoholic extract of tribulus relieved cisplatin-induced renal toxicity, perhaps via diuretic effects that increase drug excretion, scavenging free radicals via increase in antioxidant enzymes, suppressing inflammatory agents, and acting on organic cation transporter 2 (OCT2) proteins ⁽⁴¹⁾.

Tribulus extract was shown to limit formation of calcium oxalate and calcium hydrogen phosphate dihydrate crystals ^(21) (22), mineral compounds that can cause kidney stones.

Consumption of tribulus causes motor neuron adverse effects in animals by affecting the gamma-aminobutyric acid (GABA) receptors ⁽¹⁸⁾. The steroidal saponin diosgenin is thought to be responsible for hepatotoxic effects associated with tribulus ⁽²⁷⁾.