Makatoxin-3 (a.k.a. MkTx-3, MKTX III or Makatx III) is an α-like scorpion neurotoxin found in the venom of Olivierus martensii. Makatoxin-3 both enhances the activation and slows down the inactivation of voltage-gated NaV1.7 channels, resulting in hyperexcitability of the neurons involved in pain perception.[1][2]
Chemical structure of Makatoxin-3
Source
Makatoxin-3 is a neurotoxin that can be found in the venom of the scorpion Olivierus martensii, also known as Buthus martensii Karsch (BmK) or the Chinese Scorpion.[3] This scorpion is also commonly known as the Chinese scorpion, as it is widely found in China, as well as Korea and Mongolia.[3] Makatoxins account for approximately 4% of the total BmK venom protein. After high temperature processing of the venom, Makatoxins make up 0.8% of the total protein.[1]
Chemistry
Makatoxin-3 is made up of 85 amino acid residues, 19 of which make up the signal peptide at the N-terminal, and four disulfide bonds.[4][5] It has a high sequence homology (90% identity) with two other toxins from the same Makatoxin protein family, Makatoxin-1 and Makatoxin-2.[2][4][5]
Two key residues of Makatoxin-3 have been identified: K9 and R58. Their substitutions K9D and R58A in mutated Makatoxin-3 reduce the efficacy of the toxin to cause pain in mice.[2]
Makatoxin-3 is thermostable, as processing it by high temperature (60 °C) does not affect the bioactivity of Makatoxin-3 despite the protein concentration going down by 50%.[1] Makatoxin-3 is also stable in artificial gastric juice, but not in artificial intestinal juice, so Makatoxin-3 may be protected from enzymatic digestion.[1]
Makatoxin-3 shows dose-dependent effects on both the activation and the inactivation of NaV1.7.[1][2] The toxin slows the inactivation kinetics of NaV1.7 currents. The average persistent currents are larger at both low (250 nmol/kg) and high (750 nmol/kg) concentrations due to the incomplete NaV1.7 channel inactivation at depolarized potentials (-45 mV and higher). At high concentration (750 nmol/kg), the curve of the peak current shifts to the left of the hyperpolarization direction, reaching the peak earlier than in the absence of the toxin; the action potential is also prolonged by shifting the steady-state fast inactivation current to hyperpolarization potentials by -9.3 mV.[1][2] Furthermore, at high concentration (750 nmol/kg), Makatoxin-3 causes NaV1.7 to open at hyperpolarized potential by shifting its voltage-dependent activation curve by -8.6 mV.[1][2]
Toxicity
Makatoxin-3 evokes pain and allodynia.[2] After injecting 25 nmol/kg, 50 nmol/kg, and 150 nmol/kg of Makatoxin-3 respectively, mice exhibit flinching behavior due to the pain-inducing effect for approximately 30 minutes before diminishing in a dose-dependent manner (the higher the dose, the higher the number of flinches).[1]
Therapeutic use
Interestingly, notwithstanding the pain response induced by the toxin, Makatoxin-3 also shows an analgesic effect in a dose-dependent manner (50 nmol/kg, 150 nmol/kg, and 450 nmol/kg). 450 nmol/kg elicits an 80% reduction of paw-flinch pain behavior in mice when they are intraperitoneally injected with another pain-inducing substance 30 minutes after the Makatoxin-3 injection.[1] However, if the scorpion body is processed by high temperature, it requires an even higher concentration of Makatoxin-3 to elicit the analgesic effect due to the loss of these peptides.[1] A mutant variant of the neurotoxin, Makatoxin-3-R58A, still produces the analgesic effect, but largely loses its pain-inducing effect.[1][2]
Unlike morphine, which is a strong opioid receptor analgesic with the addictive side effect, the analgesic activity induced by Makatoxin-3 cannot be reversed by naloxone (μ-opioid receptorantagonist). This means that Makatoxin-3 has an analgesic effect that is not dependent on the endogenous opioid system.[1] Therefore, Makatoxin-3 is being studied as a new potential painkiller.[1]
