Sterol requirement

The formation of a complex with membrane sterols requires penetration of the glycoside into the membrane (corresponding blocks are connected by an arrow). Owing to the ability of an aglycon to form a complex [175] the corresponding block is connected with the structural elements related to an aglycon part of the molecule. The stereochemistry of lanostane nucleus plays a major role in the formation of this complex, besides, the presence of 18(20)-lactone is also important [9, 42] (these functional relations are depicted by bold arrows). 

Neurons rely upon a ready supply of cholesterol for maintaining a broad array of physiological functions such as membrane synthesis, myeUn maintenance, electrical signal transduction, synaptic transmission, and plasticity. Cholesterol metabolism in the CNS is unique compared with the rest of the body. Because of the existence of the blood-brain barrier (BBB), almost all the sterol required for new membranes comes from de novo synthesis within the CNS [33]. In addition, the brain has evolved highly efficient mechanisms to maximize the utihzation of cholesterol. UnUke other membrane lipid components, cholesterol cannot be synthesized at neuronal terminals. Therefore, synaptic function depends largely on cholesterol supplied from either axonal transport from the cell body and or uptake of Upidated ApoE produced by astroglia via neuronal lipoprotein receptors. 

Potential research topics. Because of their steroidal structure, the brassinosteroids offer many potential avenues for research both in phytochemistry and zoochemistry. If, for example, the presence of the brassinosteroids is relatively ubiquitous in plants, is it then possible that they play a role in host parasite relationships It is of note that many plant parasitic and intestinal nematodes must obtain their cholesterol precursors from their respective hosts (18), a paradigm of the former being the plant gall inducing nematode Ditylenchus dipsaci and of the latter the somewhat large intestinal nematode Ascaris lumbricoides. Certain phytopathogenic fungi also have a sterol requirement for completion of their life cycle (19), as do certain insects (20). 

Insects, unlike most vertebrates and plants, lack the capacity for de novo sterol synthesis and require dietary sterol for their normal growth, development and reproduction. This sterol requirement is in most cases satisfied by cholesterol (86) which is one of the principal sterols in insects, serving as component of the cell membranes and as a precursor of ecdysone (107). The zoophagous species such as the house fly Mucosa domestica are unable to convert phytosterol to cholesterol. For this reason, cholesterol is an essential nutrient for these species. In phytophagous and omnivorous insects, sterols such as sitosterol (87), campesterol (88), and stigmasterol (89) are dealkylated to cholesterol. Thus, 24-dealkylation is one of the essential metabohc processes in phytophagous insects (Fig. 15). 

In the case of stigmasterol (89), the proposed mechanism in Fig. 18 was evaluated as follows. [23- H]-, [24- H]- and [25- H]stigmasterols were synthesized and the fate of the deuterium atoms during the dealkylation was followed by mass spectrometry. The transfer of the deuterium atom from C-25 to C-24 was established in silkworm larvae [162]. Also, the chemically synthesized (24F)- and (24Z)-A -dienes (93 and 94) were found to satisfy the sterol requirement of the silkworm. The A -diene (96) and desmosterol (91) were identified in significant amounts from the insects in accord with the previous observation by the Beltsville group. However, mass fragmentographic analysis of the sterols of insects fed on stigmasterol (89), the... 

The sterol requirements of invertebrates are frequently satisfied by modification of dietary steroids. Thus, cholesterol is formed from 24-alkylated steroids, such as ergosterol and /5-sitosterol, by Crustaceans and insects. The mechanism of this process seems to be the reverse of their mode of formation. The 24-ethyl group of -sitosterol is converted into a 24-ethylidene group with fucosterol, and cholesta-5,24-dienol is formed on loss of the alkyl group. Cholesterol is required in insects for metabolism to the hormone ecdysone (84). However, plants also produce ecdysone and both organisms metabolize cholesterol to ecdysone. which is then further metabolized to ecdysterone (85)." ... 

Moreover, endogenous lanosterol in a heme deficient mutant of cerevisiae is detrimental to cell growth, and adaptation for growth on media supplemented with cholesterol is correlated with the absence of lanosterol (83). Bloch (84) postulated that the reason lanosterol cannot fully satisfy the sterol requirement for growth is that the axial C-14 methyl group interferes with van der Waals interactions of the sterol with fatty acyl chains of membrane lipid. Furthermore, the double bond present in C-14 methyl... 

Studies on the fat-soluble substances required by the beetles have been most interesting. FraenkeF and Fraenkel and Blewett made an extensive investigation of the sterol requirement of several beetles. They found that all five beetles studied required a sterol, although some appeared to be less adversely affected on sterol low diets than others. It is of interest that,... 

ERG5,7 = ergosta-S,7-dienol STIG 5,7,22 = Stigmesta-5,7,22-trienol REQ = sterol required, none synthesized TETRA = tetrahymenol Toitative idaitificadon... 

Sterol-requiring strain. Also contains 2% unidentified fatty acids, with retention time on GLC between those of 18 1(9) and 18 1(9,12), and 4.4% unidentified unsaturated C30 fatty acids. 

Sterol-requiring strain. 2% unidentified fatty acids present. 

Clearly, much work is required before any definite conclusions can be drawn about sterol metabolism in sponges. The exogenous sterol requirement is a characteristic of many lower organisms, the majority being invertebrates. Within the invertebrates, the position is still rather confusing as to which members can or cannot synthesize sterols. Literature reports are often conflicting, not only within the phylum, class and order, but even within the species. 

---