A Study of the Regulatory and Environmental Factors Affecting Trimethylamine Oxide Accumulation in Marine Organisms

Abstract

Trimethylamine oxide (TMAO) was first described in marine organisms as an osmolyte, involved in the balance of water and solutes. After its discovery, it was found to be part of a subset of osmotic constituents termed counteracting solutes. These solutes exhibit stabilizing properties and can preserve protein functionality against biological and environmental perturbations. TMAO acts as a universal stabilizer, protecting macromolecular structure and function in response to numerous stressors, including urea destabilization, hydrostatic pressure, temperature and salinity. The studies presented in this dissertation address the regulatory and environmental factors affecting TMAO accumulation. Both exogenous and endogenous sources are involved in the maintenance of TMAO. Exogenous TMAO accumulates through absorption from the diet while endogenous TMAO is synthesized from dietary or cellular precursors with the flavincontaining monooxygenase trimethylamine oxidase (TMAoxi). Species without a physiologically relevant synthetic capacity are hypothesized to rely entirely on dietary contributions for accumulation. Chapter 1 examines the necessity of an exogenous TMAO source on long-term maintenance in elasmobranch species with and without the ability for endogenous synthesis. These data show that presence or absence of TMAoxi cannot be used as a proxy to determine the importance of dietary TMAO on prolonged conservation. It seems that all species, regardless of synthesizing potential, rely to an extent on contributions from the diet. Chapter 2 further examines the regulatory factors affecting TMAO. This study provides evidence for endogenous production via an understudied synthetic pathway whereby TMAO is accumulated as a byproduct during lipid storage. The existence of this pathway is supported by a correlation between TMAO content and total lipid in a variety of Hawaiian mid-water fishes. The regulatory role of evolutionary relatedness on accumulation potential is also addressed in this chapter. Phylogenetic independent contrasts (PIC) showed no relationship between phylogeny and TMAO content across 27 species spanning nine orders. This suggests that environmental factors impart a larger influence on TMAO retention than evolutionary history. Chapter 2 goes on to examine TMAO’s role in combatting the environmental stress associated with increasing hydrostatic pressure. TMAO was shown to increase with increasing depth of occurrence across all species of Hawaiian mid-water fishes studied. These data support previous reports of TMAO accumulation as an environmental adaptation to combat the destabilizing effects of elevated hydrostatic pressure. Chapter 3 explores TMAO’s ability to counteract environmental fluctuations in temperature. Previous in vitro studies showed intracellular transport and accumulation of TMAO with increasing temperature in elasmobranch red