Here is an interesting finding from K. Okumura et al (2002, Archives of Biochemistry and Biophysics 408: 124–130) that may shed light on the origin of venom. Their findings contradict the assumption by Fry et al. that biochemical similarity is irrefutable evidence of homology. They wrote:

"Phospholipase A2 (PLA2) enzymes catalyze the hydrolysis of the acyl ester bond at the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. ...Snake venom is one of the most abundant sources of secretory PLA2s. Elapidae venom contains group I PLA2s and Viperidae venom, group II PLA2s. Apart from their catalytic function, these snake venom PLA2s exhibit a wide variety of pharmacological activities including neurotoxicity and myotoxicity . Venomous snakes have many serum proteins that neutralize the venom proteins presumably in order to protect themselves from the leakage of their own venom into the circulation . PLA2 inhibitory proteins (PLIs) are one of these antitoxic proteins that inhibit the toxic activities of venom PLA2s as well as their enzymatic activities."

They continued: "PLIalpha inhibits specifically group II acidic PLA2s...has been purified solely from Viperidae snakes...but not from Elapidae snakes. ...PLIbeta inhibits selectively group II basic PLA2s...has been identified only in Agkistrodon blomhoffii siniticus."

Venomous snakes use PLI to neutralize the toxic effects of the PLA2s found in their own venom. Therefore it is a surprise when Okumura et al. isolated PLIbeta from the serum of a nonvenomous snake, since this protein specifically inhibits the activity of an enzyme that "are found in PLA2-rich tissues, such as the venom glands of venomous snakes" but such tissues "are not present in the nonvenomous E. quadrivirgata." What is a PLI protein doing in the blood of a nonvenomous snake, where it does not belong? The authors noted that “Since E. quadrivirgata often feed on the Japanese mamushi (A. halys blomhoffii), we cannot exclude the possibility that E. quadrivirgata PLIs might also function as defense proteins in order to protect themselves from envenomation by the venomous snakes.” That is certainly one possibility, but even this possibility means that very similar molecules can apparently evolve independently in two unrelated snake lineages, proving once again that biochemical similarity is not irrefutable evidence of homology, as it had been previously proven in the evolution of the eye.

Okumura further analyzed the prevalence of PLI beta in Elaphe quadrivirgata and found that it was “detected in liver samples and at a lower level in lung samples of the snake” but not in “...other tissues such as spleen, pancreas, fat body, small intestine, gallbladder, testis, kidney, esophagus, and heart.” They concluded that “EqPLIbeta expressed in the lung might have some important physiological
functions in regulating local PLA2 activity.”

This paper therefore shows that group 2 PLA2, which is present in large quantities in viperid snake venom but not in elapid snake venom, is surprisingly found in the lung, liver and blood of a nonvenomous snake Elaphe quadrivirgata. Its presence in the lung may, according to the authors, peform some unknown but important physiological function. If that is the case, group 2 PLA2 may indeed be widespread among snakes. Its presence in viperid snake venom but not in elapid snake venom shows that venom probably evolved independently in these two lineages of snakes. It also shows that many toxic substances found in Duvernoy's glands may have other functions unrelated to a possible role as venom, as Kenneth Kardong suggested. Therefore the presence of these toxic substances is not prima facie evidence that a snake is either venomous or a secondarily non-venomous descendant of a venomous ancestor, contra the claims of Fry et al. and others.

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