The origin of vertebrate steroids in molluscs: uptake, metabolism and depuration studies in the common mussel
Year: 2016
Type: dissertation
Abstract: Many studies have found vertebrate sex steroids, such as testosterone (T), 17β-oestradiol (E 2 ) and progesterone (P) to be present in molluscan tissues.The underlying assumption of most, if not all, these studies has been that these steroids are formed endogenously and furthermore, act as reproductive hormones in the same way that they do in vertebrates (i.e. they bind to receptors and induce physiological processes such as egg yolk protein production in females, sex reversal in bivalves and penis formation in neogastropods).However, an in depth evaluation of the literature indicates that the evidence for endogenous formation of vertebrate steroids is rather weak (for example, mollusc genomes do not contain sequences for critical P450 enzymes such as 17-hydroxylase and aromatase nor for functional steroid nuclear receptors).The evidence for the uptake of at least two of these steroids, 17β-oestradiol (E 2 ) and testosterone (T) from the environment appears, on the other hand, to be very strong and the aim of this thesis was to study this in more detail in common mussels (Mytilus edulis and a mixed population of M. edulis / Mytilus galloprovincialis).This involved exposing mussels to tritiated E 2 , T, progesterone (P), 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), 17αethinyl-oestradiol (EE 2 ) and cortisol (F) and then determining the rate and capacity of uptake, how much was esterified (i.e.conjugated to fatty acids), how much if any was sulphated and finally, how rapidly it was depurated.The study also required novel procedures to be developed for optimising in vivo radiolabel uptake and for separating free (i.e.nonconjugated), esterified and sulphated steroids.It was discovered that all steroids except, intriguingly, F, were rapidly absorbed by mussels from the water.It was discovered that the rate of uptake of radioactive steroids could not be saturated by the addition of non-radioactive steroid (even at 25µg L -1 ), i.e. the uptake capacity was far higher than amounts found in nature.It was found that the largest proportion (> 70 %) of radioactive E 2 and T in the tissues was present as fatty acid esters.These depurated very slowly (E 2 had a half-life of ca. 12 days, while T showed no signs of depletion after 10 days).It was found that the level of esterification of EE 2 , P and 17,20β-P was noticeably lower than that of E 2 and T. Following saponification of esterified E 2 , a single steroid (E 2 itself) was recovered.However, following saponification of esterified P, two steroids, neither of them P and not yet fully identified, were recovered.Following saponification of T, four steroids were recovered.The least abundant (< 10 %) was T itself and the most abundant was 5α-dihydrotestosterone (47 %).It was discovered that sulphation played an important role in the metabolism of E 2 .While none of these results disprove endogenous synthesis of vertebrate steroids, they do call into question those studies that have claimed a cause and effect relationship between vertebrate steroid concentrations in mollusc tissues and factors such as the stage of the reproductive cycle, gender or exposure to presumed 'endocrine' disrupters (i.e. it is impossible to determine in any of these studies whether any of the steroids were of endogenous origin).
Author: Tamar Schwarz
Language: English
Subfield: Physiology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2475203666