An analysis of venom ontogeny and prey-specific toxicity in the Monocled Cobra (Naja kaouthia)

Abstract

Venoms of snakes of the family Elapidae (cobras, kraits, mambas, and relatives) are predominantly composed of numerous phospholipases A2 (PLA2s) and three-finger toxins (3FTxs), some of which are lethal while others are not significantly toxic. Currently, the only identified prey-specific toxins are several nonconventional 3FTxs, and given the large diversity of 3FTxs within Monocled Cobra (Naja kaouthia) venom, it was hypothesized that several 3FTxs, previously found to be non-toxic or weakly toxic 3FTxs in murine models, could potentially be toxic towards non-murine prey. Additionally, it was hypothesized that ontogenetic dietary shifts will be correlated with observable changes in specific 3FTx isoform abundance. Adult and juvenile N. kaouthia venom composition was investigated using ion-exchange FPLC, 1D and 2D SDS-PAGE, mass spectrometry, and various enzymatic and LD50 assays. Alpha-cobratoxin (α-elapitoxin) was the only significantly toxic (LD50 < 1 μg/g) 3FTx found in N. kaouthia venom and was equally toxic toward both lizard and mouse models. The abundance and diversity of 3FTxs and most enzyme activities did not vary between adult and juvenile cobra venoms; however, total venom PLA2 activity and specific PLA2 isoforms did vary, with juveniles lacking several of the least acidic PLA2s, and these differences could have both biological (related to predation) and clinical (antivenom efficacy) implications. Nevertheless, the ubiquitous presence of α-cobratoxin in both adult and juvenile cobra venoms, with high toxicity toward both reptiles and mammals, represents a venom compositional strategy wherein a single potent toxin effectively immobilizes a variety of prey types encountered across life history stages.