Pharmacodynamics of Snake Venoms and Envenomation
by
Lee Moore
Introduction 

This paper is not intended as an all-inclusive presentation of all components of snake venoms nor does it propose to explain all the physiological processes involved in snake envenomation. Rather it is intended as a brief introduction and description of the biochemical mechanisms involved during snake envenomations in general. It attempts this from an innovative and simplistic perspective.

It should be pointed out here, that the previously established practice of classifying a venom as either hematoxic or neurotoxic has proven to be flawed and insufficient in defining the true nature of snake venoms and toxins. Since the establishment of the International Society on Toxinology and publishing of its interdisciplinary journal of toxins, TOXICON,   there has been a source of shared information that has helped to provide some standardization. However, there is still a problem with general classifications. Toxins are often termed according to the emphasis of the study in which they are identified.  Cardiotoxins, as an example, can be a cytotoxin that has proven to have considerable effects on the cardiac muscle cells, or a Myotoxin that specifically affects the heart.  It may be that these generalizations cannot be avoided or may even be necessary since the effects of toxins are certainly dependant on a number of factors exogenous and endogenous to the physiology of the host. Toxins cannot be expected, except under controlled situations, to always react with the same effects. However with this generalization of terms it is necessary to have a broad knowledge of all the potential specifics.

Snake venoms are composed of various collections of polypeptides. These polypeptides are toxins that or either enzymes or non-enzymatic polypeptides. The effects or actions of these toxins are, on the most part, either by a means of degradation of cells, tissues and/or intercellular bonds or by a competitive inhibition blocking transmission of acetylcholine. Exceptions to this is found in all species belonging to the genus Dendroapsis, snakes known as mambas.

Other venom components have been identified including carbohydrates, lipids, nucleoside and some metals. Magnesium, calcium, and zinc are the most prevalent metals, copper has been detected in some venoms. There are studies that suggest that some of these co-constituents may play an active role in the lethality of venoms.  The purpose of this paper is not to present validity to, or to dispute any such finding. However, it is the opinion of this author, that though there probably is validity in these findings the importance of these substances to effect toxicity would probably be academic, because such substances are either endogenous to the host or indigenous, in some way, to the envenomation itself.

Enzymes

Though most snake venoms have enzymes that act as toxic constituents, all are composed of several enzymes. More than 20 enzymes have been detected in snake venoms, and 12 are found in all venoms. The majority of toxic effects of viperid and crotalid (pit vipers, including rattlesnakes moccasins, etc.) envenomations are due to enzymes collectively termed “proteases”. These are actually hyrdolases that primarily act to breakdown proteins, by hydrolysis and thus are proteolytic and serve a digestive role.  They are also the cause of most of the local damage following envenomation. These enzymes are further separated and classified according to the mode or target site of their actions, for instance endo - and exopeptidases. These target bonds in the peptide chains that hold proteins together. One of the enzymes that are present in all snake venom is Hyalurondase. It is present in viperid, crotalid, elapid and hydrophid venoms, as well as in most animal venoms. Its effect can readily be observed following envenomation as blood oozing from the bitten site. This is the first good indication that can be observed, following a bite, to determine if an envenomation has occurred. The absence of this oozing is a possible indication that no or little venom was injected.  Hyalurondase is known to have a “spreading action”. It is the first enzyme to take action by facilitating other venom components through tissues and into the bloodstream. It greatly increasing permeability of membranes, ruptures cellular walls and alters coagulation. In a sense it acts as a “ vehicle” to provide active transport for other toxic components.

It is not only the snake toxins itself that exerts toxic effects, but also the products that are produced by the digestive process of the enzymes. Such is the case involving the phospholipases that produces lytic products by cleaving membrane-bound phospholipids.  Another example of this involves L-Amino-acid oxidase, which is another found in all snake venoms. It causes the release of the powerful lytic chemical hydrogen peroxide. Its toxic effect is by oxidation of cells.

In cases involving venoms that exerts their primary toxic effects as a neurotoxin,  affecting neuromuscular processes, it is not  enzymes, but other non–enzymatic polypeptides that act as the primary lethal agent. However, enzymes, always play an active role in facilitating the spread of  these non-enzymatic polypeptides.

PROTEOLYTIC OR HEMOLYTIC?

Snake venom enzymes are proteolytic and hemorrhagic toxins.  These cause hemorrhage by targeting fibrinogen converting it by digestion into an unclottable substance or by directly causing lysis of blood cells. Blood platelets and Prothrombin are also sometimes affected leading to coagulopathy of sorts and even hemorrhage. Ultimately hypovolemia can result in any case.  Constituents have been found in snake venoms that have activating and inactivating effects on nearly all aspects of hemostasis. Some studies have indicated that some of these hemorrhagic toxins may not be enzymatic in nature. However, most are certainly enzymes. In contrast to causing hemorrhage, some toxins has been found to act in the same path as human endogenous clotting factors, but in the cases of these exogenous agents there are no controlled corresponding inhibitors. A notable example is the Russell’s viper Daboia Russelli.  Its venom causes severe coagulopathy by preventing coagulation then activating uninhibited coagulation. Much research using this venom has already revealed uses to further understand human clotting factors and control coagulopathy disorders.

The method by which proteolytic toxins digest proteins is not entirely different than what occurs during hemolysis. In both cases there is lysis causing cellular and tissue destruction by the some means of degradation.  The question, or rather an observation may arise, what differentiates a proteolytic toxin as opposed to a hemolytic one? Perhaps what actually constitutes the difference is the level at which the lytic damage is noted.

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