R.C.H.M. Oudejans

Fatty acid synthesis in relation to gametogenesis in the mussel Mytilus edulis L.

1. 1. Seasonal fluctuations of the lipid level in Mytilus edulis are small compared to the changes in proteins and glycogen. 2. 2. In the later stages of gametogenesis (March–May) there is an increase of neutral lipids in the mantle, while there is no concomitant decrease elsewhere in the animal. 3. 3. Fatty acid analysis shows an increase of probably de novo synthesized fatty acids. 4. 4. Activity of acetyl-CoA carboxylase and fatty acid synthetase are virtually absent in mantle, muscle and digestive gland. 5. 5. These enzyme activities increase strongly in female mantle tissue in the period preceding the spawning, but only slightly in male mantles.

Adipokinetic peptide hormone content and biosynthesis during locust development

The content and biosynthesis of adipokinetic hormones (Lom-AKH-I, -II. and -III) were studied in larval stages and adults of Locusta migratoria. The amount of all three AKHs increases with age, although the patterns found for AKH-I and -II differ from that for AKH-III. Biosynthetic capacity of the corpus cardiacum for the three AKHs increases with age, particularly in larvae, whereas in adults this increase is only observed for AKH-III. The amounts of AKH-I and -II stored and their active biosynthesis greatly surpass the small quantities needed for mobilization of fuels during flight. The data for AKH-III suggest that this hormone may be important also during larval stages.

Differential location of peptide hormones in the secretory pathway of insect adipokinetic cells.

Abstract. Immunoreactivity of granules containing secretory material in the adipokinetic cells of the insect Locusta migratoria was studied using antisera specific for the adipokinetic hormone-associated peptides (AAP) I, II and III. Immunocytochemical detection of these associated peptides represents a new strategy for studying the intracellular location of the adipokinetic hormones and their prohormones. Fixation with 2% glutaraldehyde and 2% formaldehyde with low-temperature embedding in Lowicryl HM20 allowed highly selective immunogold labelling of both secretory and intracisternal granules. All three associated peptides were co-localized in secretory granules. This indicates that also all three adipokinetic hormones can be co-localized in these granules, which was confirmed by experiments in which, after secretory stimulation, adipokinetic hormone III was released from the adipokinetic cells together with adipokinetic hormones I and II. The immunopositivity of the intracisternal granules was similar to that of the secretory granules, although with the exception that the intracisternal granules did not show any specific reaction with anti-AAP III. The presence of AAP I and AAP II in intracisternal granules indicates that these granules only function as stores of adipokinetic prohormones I and II and not of adipokinetic prohormone III. The observed differences in storage in intracisternal granules among the three adipokinetic prohormones suggest differences in physiological significance of the three adipokinetic hormones in L. migratoria.

Aerobic-anaerobic biosynthesis of fatty acids and other lipids from glycolytic intermediates in the pulmonate land snail Cepaea nemoralis (L.)

Abstract 1. 1. Aerobic-anaerobic lipid biosynthesis was studied in the snail Cepaea nemoralis after injection of 14C-labelled glucose, pyruvate, alanine, lactate and acetate. 2. 2. Anaerobic incorporation of acetate and pyruvate into the total lipids was fully comparable to that under aerobic conditions, thus implying an anaerobic decarboxylation of pyruvate and utilization of acetyl-CoA in fatty acid synthesis. 3. 3. Parts of the radiolabel from end products of anaerobic glycolysis, such as alanine and lactate, were incorporated into fatty acids and other lipids during anoxia. 4. 4. The generally accepted specificity of molluscan lactate dehydrogenase for d -lactate was not confirmed in Cepaea. 5. 5. Some coupling of anaerobic glycolysis and anaerobic fatty acid biosynthesynthesis may be explained in terms of a reoxidation of part of the glycolytic formed NADH or NADPH.

Effect of excessive fatty acid ingestion upon composition of neutral lipids and phospholipids of snailHelix pomatia L.

The effect of an excessive intake of oleic acid on the lipids of the Roman snail (Helix pomatia L.) was studied. The total lipid content increased by 30% which was fully attributable to a marked elevation in the neutral esters and free fatty acids, as phospholipid and free sterol contents remained unaffected. The fatty acid composition of the phospholipids, characterized by high amounts of stearic, linoleic, homolinoleic, and, particularly, arachidonic acids, appeared to be nearly insensitive to this excessive oleic acid ingestion. By contrast, the effect of oleic acid upon the depot lipids was striking: active intestinal resorption of the acid from the dietary supply was shown by the fourfold level of lleic acid in the free fatty acid fraction, whereas a fivefold level of this acid in the glyceride and sterol ester fraction was proof of a substantial esterification. These data support the view that the composition of the structural lipids is specifically species oriented, whereas both the content and the composition of the depot lipids are highly governed by dietary fat intake.

Absence of coupling between release and biosynthesis of peptide hormones in insect neuroendocrine cells

Adipokinetic hormone (AKH)-producing cells in the corpus cardiacum of the insect Locusta migratoria represent a neuroendocrine system containing large quantities of stored secretory peptides. In the present study we address the question whether the release of AKHs from these cells induces a concomitant enhancement of their biosynthesis. The effects of hormone release in vivo (by flight activity) and in vitro (using crustacean cardioactive peptide, locustamyoinhibiting peptide, and activation of protein kinase A and C) on the biosynthetic activity for AKHs were measured. The intracellular levels of prepro-AKH mRNAs, the intracellular levels of pro-AKHs, and the rate of synthesis of (pro-)AKHs were used as parameters for biosynthetic activity. The effectiveness of in vitro treatment was assessed from the amounts of AKHs released. Neither flight activity as the natural stimulus for AKH release, nor in vitro treatment with the regulatory peptides or signal transduction activators appeared to affect the biosynthetic activity for AKHs. This points to an absence of coupling between release and biosynthesis of AKHs. The strategy of the AKH-producing cells to cope with variations in secretory stimulation seems to rely on a pool of secretory material that is readily releasable and continuously replenished by a process of steady biosynthesis.

In Vitro Biosynthesis of Locust Adipokinetic Hormones: Isolation and Identification of the Bioactive Peptides and Their Prohormones

The biosynthesis of adipokinetic hormones (AKH) I and II and their precursors was studied in the migratory locust Locusta migratoria using radiolabeled amino acids in an in vitro system. The peptides from the glandular part of the corpora cardiaca (CC) were extracted and separated by reversed-phase high performance liquid chromatography (HPLC). Incorporation of the [3H] amino acids was measured by monitoring the radioactivity of the column eluate. Total time for biosynthesis and processing of the prohormones to the bioactive AKH I and II was 75 min. Only the glandular part of the CC synthesized these peptide hormones.

Fatty acids of the female horseshoe crab Xiphosura (Limulus) polyphemus

The fatty acid composition of 9 different tissues and organs of the female horseshoe crab Xiphosura (Limulus) polyphemus — one of the very few recent representatives of the ancient arthropod class Merostomata — was investigated in reference to the distribution of fatty acids through the marine food web. Fatty acid spectra, in which polyunsaturated fatty acids are predominant, especially eicosapentaenoic acid (20:5 ω3), revealed features characteristic of marine lipids. However, rather large quantities of monoenoic fatty acids also occur in all organs. In the saturated fatty acid fractions, the high content of branched-chain components is worth noticing, particularly in the gills and the carapace (35%); in all probability, the high amount of the branched-chain fatty acids is associated with their protective function in surface lipids. Isoprenoid fatty acids such as pristanic and phytanic acid were absent.

Coherence between biosynthesis and secretion of insect adipokinetic hormones

The importance of the process of continuous biosynthesis of locust adipokinetic hormones (AKHs) for the availability of these peptide hormones for release was assessed in vitro by inhibiting this biosynthesis followed by secretory stimulation. Inhibition of the biosynthetic activity for AKHs by brefeldin A caused a considerable inhibition of the AKH release induced by the endogenous crustacean cardioactive peptide (CCAP). After brefeldin A treatment followed by potassium depolarization, CCAP-induced AKH release was completely abolished. In vitro pulse-chase labeling experiments indicated that constitutive secretion from the AKH-producing cells does not occur. It is concluded that AKH secretion involves a regulated release from a relatively small pool of newly formed secretory granules, while older AKH-containing granules appear to be unavailable for release.

Adipokinetic Hormones: Coupling between Biosynthesis and Release

Abstract: During long‐distance flight of migratory locusts, the dramatic energy demand of the flight muscles is controlled by three adipokinetic hormones (AKHs). These peptide hormones regulate the mobilization of lipid and carbohydrate stored in the fat body to serve as energy substrates for the flight muscles. Despite the relatively huge quantities of the three AKHs that are stored in the corpora cardiaca, flight induces a differential 2‐4‐fold increase in the mRNAs for the three hormones. Moreover, newly synthesized AKHs can be released only during a restricted period of time, suggesting that by far most of the stored hormones are physiologically inactive. This raises the question of how the biosynthetic activity in the AKH‐producing cells is coupled to their secretory activity. The present review discusses the potential mechanisms by which generation and release of mixtures of bioactive neurohormones are controlled and how peptidergic neuroendocrine cells cope with variations in physiological stimulation, with the AKH‐producing cells serving as a model system.