**1. Introduction**

458 Drug Discovery

Snake venoms contain a complex mixture of proteins and biologically active peptides [1, 2]. Some of these bioactive peptides are derived from precursor proteins that through proteo‐ lytic processing generate mature active polypeptides [3]. As an example, the protein precur‐ sor of natriuretic peptide type-C (CNP) from the Brazilian pit viper *Bothrops jararaca* venom and brain originates CNP, a hormone present in several animal species as well as various isoforms of proline-rich oligopeptides (*Bj*-PROs) [4, 5]. *Bj*-PROs were the first natural inhibi‐ tors of angiotensin I-converting enzyme (ACE) described [6]. The metalloproteinase ACE, the key enzyme of the renin-angiotensin system, displays two homologous active sites, one at the C-terminal and the other at the N-terminal of the protein [7]. While both active sites convert angiotensin I into angiotensin II and cleave bradykinin (BK) into BK1-5 and BK1-7, the C-terminal is more effective in hydrolysis of these vasoactive peptides [8].

*Bj*-PROs are molecules of 5 to 14 amino acids residues with a pyroglutamyl residue (<E) at the N-terminus and a proline residue at the C-terminus. *Bj*-PROs longer than seven amino acids share similar features, including a high content of proline residues and a C-terminal tripeptide sequence Ile–Pro–Pro [13]. Since *Bj*-PROs are ACE inhibitors, they potentiate some pharmacological activities of BK, such as induction of contractile action of smooth muscles of guinea-pig ileum *in vitro* as well as *in vivo* BK-induced effects on central nervous, cardiovascular and anti-nociceptive systems [6, 9, 10]. For this reason, these peptides were initially named bradykinin-potentiating peptides (BPPs). The ability of some *Bj*-PROs inhib‐ iting ACE turned them in structural models to develop the first non-peptide site directed in‐ hibitor of this enzyme. The development of Captopril in the early 1980s became a paradigm

© 2013 Lameu and Ulrich; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

for "rational drug design", a concept much heralded today and made possible by computer imaging and genome science [11].

three major different functions in the adult organism depending on the cell/tissue consid‐ ered: (i) ammonia detoxification in the liver, (ii) L-arginine production for the whole organ‐ ism by kidney and (iii) L-arginine synthesis for NO production in many other cells [22].

Applications of Snake Venom Proline-Rich Oligopeptides (Bj-PROs) in Disease Conditions Resulting from...

Three isoforms of NOS catalyze the reaction: the endothelial constitutive NOS (eNOS), the neuronal constitutive NOS (nNOS) and the inducible NOS (iNOS), reviewed in [29, 30]. NO is a gaseous molecule capable of interacting with many intracellular targets for triggering a series of signal transduction pathways, resulting in a stimulatory and inhibitory signals. NO plays roles in cardiovascular, immune and neuronal control. It is directly involved in arterial tension control since it regulates the local and systemic resistance of vascular walls, as well as the sodium balance [31]. The NO produced by endothelial cells reaches neighboring

pendent [32, 33], and thereby induces the relaxation of blood vessels and brings about vas‐ cular dilation leading direct consequences in processes like erection, arterial pressure

Due to the great physiological importance of the NO, compounds revealing properties aspo‐ tential NO donors have been protected by patents. They are based on the fact that there are many pathological states related to NO deficiency (reviewed by [35]). However, a major problem of NO donors is to achieve a therapeutic dose without reaching a threshold of tox‐ icity. Mostly, if NO is produced in excess around of cells in a pro-oxidant state, NO could react with reactive oxygen species (ROS) such as superoxide and hydrogen peroxide form‐ ing peroxynitrite and nitrogen oxide III, which has been linked to pathogenesis of neurode‐

The superoxide anion is produced by uncoupling of NOS due to the lack of its natural sub‐ strate L-arginine or tetrahydrobiopterin (BH4) [36], an important cofactor for NOS. Excessive production of superoxide is explained by increased activity and expression of the enzyme arginase, which competes with eNOS for its substrate L-arginine [37]. Other possible sources for elevated concentrations of ROS include increased expression and activity of NADPH

In order to compensate for the deficiency of NO production without induction of toxicity, addition of exogenous L-arginine in the maintenance of NO production has been investigat‐ ed. The inefficiency of swallowed L-arginine in promoting increase of NO can be explained by its low availability, due to the first pass effect, since the viability of L-arginine as sub‐ strate for NOS is reduced by the activity of arginase in the liver. Several studies have shown that induction or activation of arginase may lead to impaired NO production and endothe‐

Thus, NO presents challenges and opportunities to intervene and promote human health. The study of regulation of NO production becomes important for understanding the mecha‐

An important mechanism for control and maintenance of NO levels is achieved by its recy‐ cling via the NO-citrulline cycle. The obtained L-arginine provided by the citrulline-NO cy‐ cle is then directed to sustain NO production, sparing bulk intracellular L-arginine for other

nisms which maintain NO levels in a safe range and not injurious to the body.

channels, ATP-sensitive and Ca2+-de‐

http://dx.doi.org/10.5772/52509

461

smooth muscle cells where it activates two types of K+

and reduced superoxide dismutase activity [38, 39].

lial dysfunction (reviewed by [35]).

systemic or organ-specific [34].

generative disorders [35].

Many studies on structure–activity of *Bj*-PROs showed that a simple analogous structure to Ala–Pro was optimal for binding to the active site of ACE. Replacement of the carboxyl by a sulfhydryl group enhanced the inhibitory activity of the analogue by 1,000-fold. This com‐ pound proved to be one of the most potent competitive inhibitors of ACE and, therefore, turned into a useful drug to treat human hypertension (reviewed by [12]). Captopril was a blockbuster drug and inspired the creation of several generations of similar antihyperten‐ sive compounds.

However, due to structural diversity of *Bj*-PROs [13] other mechanisms besides inhibiting ACE were proposed. In fact, some of these peptides augmenting argininosuccinate synthase (AS) activity *in vitro* and *in vivo*, can also induce rises in free intracellular calcium concentra‐ tion ([Ca2+]*<sup>i</sup>* ) by acting on muscarinic acetylcholine, BK or yet unidentified receptors [15-18] or reversal inhibition of nicotinic acetylcholine receptor [19]. These novel mechanisms of ac‐ tion, recently identified for *Bj*-PROs explain their anti-hypertensive effects [14-17, 20, 21]. Therefore, investigation of *Bj*-PRO-induced effects through acting on different targets opens possibilities of applications for these peptides in the treatment of several pathologies lacking efficient treatment options.

Here, we describe the targets of various *Bj*-PROs and their potential use to treat different target-related pathologies, as well as discuss chemical properties of these peptides for ob‐ taining an oral pharmaceutical formulation.
