Reason for indispensability of threonine in humans and other mammals in comparative aspect

The essential amino acid threonine is not synthesized in vertebrates, so it must be obtained from food. During evolution, the decomposition of threonine has changed. Because the decomposition of threonine catalyzed by threonine dehydratase is irreversible, in the present work attention is focused on threonine dehydrogenase to show the inability of this enzyme to synthesize threonine in a reaction that would be the reverse of the reaction of threonine decomposition. The reason why threonine dehydrogenase cannot be used for the biosynthesis of threonine in mammalian tissues is discussed. It is concluded that some quantity of threonine is involved in transamination.     

Regulated externalization of phosphatidylserine at the cell surface: implications for apoptosis

The regulated loss of plasma membrane phosphatidylserine (PS) asymmetry is critical to many biological processes. In particular, the appearance of PS at the cell surface, a hallmark of apoptosis, prepares the dying cell for engulfment and elimination by phagocytes. While it is well established that PS externalization is regulated by activation of a calcium-dependent phospholipid scramblase activity in concert with inactivation of the aminophospholipid translocase, there is no evidence indicating that these processes are triggered and regulated by apoptotic regulatory mechanisms. Using a novel model system, we show that PS externalization is inducible, reversible, and independent of cytochrome c release, caspase activation, and DNA fragmentation. Additional evidence is presented indicating that the outward movement of plasma membrane PS requires sustained elevation in cytosolic Ca2+ in concert with inactivation of the aminophospholipid translocase and is inhibited by calcium channel blockers.     

Oxidative signaling pathway for externalization of plasma membrane phosphatidylserine during apoptosis

Active maintenance of membrane phospholipid asymmetry is universal in normal cell membranes and its disruption with subsequent externalization of phosphatidylserine is a hallmark of apoptosis. Externalized phosphatidylserine appears to serve as an important signal for targeting recognition and elimination of apoptotic cells by macrophages, however, the molecular mechanisms responsible for phosphatidylserine translocation during apoptosis remain unresolved. Studies have focused on the function of aminophospholipid translocase and phospholipid scramblase as mediators of this process. Here we present evidence that unique oxidative events, represented by selective oxidation of phosphatidylserine, occur during apoptosis that could promote phosphatidylserine externalization. We speculate that selective phosphatidylserine oxidation could affect phosphatidylserine recognition by aminophospholipid translocase and/or directly result in enzyme inhibition. The potential interactions between the anionic phospholipid phosphatidylserine and the redox-active cationic protein effector of apoptosis, cytochrome c, are presented as a potential mechanism to account for selective oxidation of phosphatidylserine during apoptosis. Thus, cytochrome c-mediated phosphatidylserine oxidation may represent an important component of the apoptotic pathway.     

The concentration of carnitine in the luminal fluid of the testis and epididymis of the rat and some other mammals.

Luminal fluid was collected by micropuncture techniques from the testis and epididymis of the rat, hamster, rabbit, boar and ram and the concentration of free L-carnitine in the fluid was estimated using enzymic methods. Carnitine was present in the testicular fluid of the rat in concentrations less than 1 mM but increased down the epididymis to reach 53 mM in luminal fluid from the cauda epididymidis, approximately 2000 times higher than in blood plasma. A high concentration was first found in the luminal fluid from the distal caput epididymidis, at about the point where the spermatozoa become motile. Carnitine was also present in the epididymal luminal fluid of the other species studied; the amounts were not as high as those in the rat but were still higher than those in blood plasma.     

Relative testis size and sperm morphometry across mammals: no evidence for an association between sperm competition and sperm length

Understanding why there is extensive variation in sperm form and function across taxa has been a challenge because sperm are specialized cells operating at a microscopic level in a complex environment. This comparative study collates published data to determine whether the evolution of sperm morphometry (sperm total length and separate component dimensions) is associated with sperm competition (when different males' sperm mix and compete for a female's ova) across 83 mammalian species. We use relative testes mass as an indicator of the intensity of sperm competition across taxa: relative investment into testes is widely accepted to predict the level of sperm competition that a species or population endures. Although we found evidence for positive associations between relative testes mass (controlling for allometry) and sperm morphometry across 83 mammalian species, these relationships were phylogenetically dependent. When we appropriately controlled for phylogenetic association using multiple regression within a phylogenetic framework, there was no relationship between relative testes mass and sperm length across mammals. Furthermore, we found no evidence for associations between relative testes mass and sperm head, mid-piece or flagellar lengths, nor was there a relationship with mid-piece or mitochondrial volumes. Results, therefore, indicate that sperm competition does not select for longer or shorter sperm across mammals, and alternative forces selecting on sperm form and function are discussed.     

The evolutionary history of testicular externalization and the origin of the scrotum

This paper re-examines the evolution of the scrotum and testicular descent in the context of the recent phylogeny of mammals. The adaptive significance of testicular descent and scrotality is briefly discussed. We mapped four character states reflecting the position of testes and presence of scrotum onto recent mammalian phylogeny. Our results are interpreted as follows: as to the presence of testicondy in Monotremata and most of Atlantogenata, which represent the basal group of all eutherians, we argue that primary testicondy represents a plesiomorphic condition for Eutheria as well as for all mammals. This is in opposition to the previous hypothesis of Werdelin and Nilsonne that the scrotum may have evolved before the origin of mammals and then repeatedly disappeared in many groups including monotremes. We suggest that the scrotum evolved at least twice during the evolutionary history of mammals, within Marsupialia and Boreoeutheria, and has subsequently been lost by many groups; this trend is especially strong in Laurasiatheria. We suggest that the recent diversity in testicular position within mammals is the result of multiple selection pressures stemming from the need to provide conditions suitable for sperm development and storage, or to protect the male gonads from excessive physical and physiological disturbance.     

Electron microscopic evidence for externalization of the transferrin receptor in vesicular form in sheep reticulocytes

Using ferritin-labeled protein A and colloidal gold-labeled anti-rabbit IgG, the fate of the sheep transferrin receptor has been followed microscopically during reticulocyte maturation in vitro. After a few minutes of incubation at 37 degrees C, the receptor is found on the cell surface or in simple vesicles of 100-200 nm, in which the receptor appears to line the limiting membrane of the vesicles. With time (60 min or longer), large multivesicular elements (MVEs) appear whose diameter may reach 1-1.5 micron. Inside these large MVEs are round bodies of approximately 50-nm diam that bear the receptor at their external surfaces. The limiting membrane of the large MVEs is relatively free from receptor. When the large MVEs fuse with the plasma membrane, their contents, the 50-nm bodies, are released into the medium. The 50-nm bodies appear to arise by budding from the limiting membrane of the intracellular vesicles. Removal of surface receptor with pronase does not prevent exocytosis of internalized receptor. It is proposed that the exocytosis of the approximately 50-nm bodies represents the mechanism by which the transferrin receptor is shed during reticulocyte maturation.     

Phosphatidylserine externalization during differentiation-triggered apoptosis of erythroleukemic cells

K562 erythroleukemia cells undergo apoptosis when induced to differentiate along the erythroid lineage with hemin. This event, characterized by DNA fragmentation, correlated with downregulation of the survival protein, BCL-xL, and decrease in mitochondrial transmembrane potential (deltapsi[m]) that ultimately resulted in cell death. Reorientation of phosphatidylserine (PS) from the cells inner-to-outer plasma membrane leaflet and inhibition of the aminophospholipid translocase was observed upon hemin-treatment. Constitutive expression of BCL-2 did not inhibit hemin-induced alterations in lipid asymmetry or decrease in deltapsi[m], and only moderately prevented DNA fragmentation. BCL-2, on the other hand, effectively inhibited actinomycin D-induced DNA fragmentation, the appearance of PS at the cells outer leaflet and the decrease in deltapsi[m]. The caspase inhibitor, z.VAD.fmk, blocked DNA fragmentation by both hemin and actinomycin D, but inhibited PS externalization only in the actinomycin D-treated cells. These results suggest that, unlike pharmacologically-induced apoptosis, PS externalization triggered by differentiation-induced apoptosis occurs by a mechanism that is associated with a decrease in deltapsi[m], but independent of BCL-2 and caspases.     

Rapid constitutive internalization and externalization of epidermal growth factor receptors in isolated rat hepatocytes. Monensin inhibits receptor externalization and reduces the capacity for continued endocytosis of epidermal growth factor

It was previously demonstrated that freshly isolated rat hepatocytes can internalize severalfold more epidermal growth factor (EGF) molecules than the number of surface EGF receptors, suggesting extensive reutilization of receptors during endocytosis (Gladhaug, I. P. & Christoffersen, T. (1987) Eur. J. Biochem. 164, 267-275). The present report attempts to explore the pathways involved in the externalization of EGF receptors. Incubation of hepatocytes at 37 degrees C in the absence of ligand increased the surface receptor pool by 50-100% within 45 min. Pretreatment with monensin inhibited the turnover of the surface EGF receptor pool by 50-60% within 10 min and blocked the temperature-dependent externalization of receptors. Cycloheximide caused a slower attenuation of the surface receptor pool, whereas tunicamycin and chloroquine did not significantly affect the exchange of receptor pools. Monensin reduced the surface receptor pool and the endocytic uptake in corresponding proportions, without affecting the internalization of prebound EGF. Endocytic uptake was unaffected by chloroquine and slightly reduced by cycloheximide. The internalization of unoccupied receptors and the endocytosis of prebound EGF followed similar kinetics (t1/2 approximately 5 min), suggesting that unoccupied receptors are internalized at a rate comparable to that of occupied receptors. The results suggest that there is a rapid turnover of the surface pool of EGF receptors with constitutive internalization of unoccupied surface receptors and externalization of internal receptors. This is consistent with, but does not prove, a true recycling of the EGF receptors in the hepatocytes. The monensin-sensitive externalization pathway determines the capacity for continued endocytosis of EGF.     

Galactosyltransferase, pyrophosphatase and phosphatase activities in luminal plasma of the cauda epididymidis and in the rete testis fluid of some mammals

Galactosyltransferase activity was measured in the luminal plasma of the cauda epididymidis of mice, rats, rabbits, rams and boars, and in the rete testis fluid of rams and boars. The activities of nucleotide pyrophosphatase and alkaline phosphatase, which compete with galactosyltransferase for substrate, were also determined. In these species, galactosyltransferase activity in the luminal plasma of the cauda epididymidis was similar when the inhibitory effect of pyrophosphatase and phosphatase was minimized by assay conditions. However, under assay conditions that did not minimize the effect of these enzymes, the galactosyltransferase activities of these species were very different and were inversely correlated with the activities of pyrophosphatase and phosphatase. The ratio of galactosyltransferase activity to pyrophosphatase and phosphatase activity was much higher in the rete testis fluid than in the luminal plasma of the cauda epididymidis in both rams and boars. In rams, galactosyltransferase in the luminal plasma of the cauda epididymidis was more heat resistant than that in serum. These results suggest that there is a species difference in the availability of galactosyltransferase activity in the luminal plasma of the cauda epididymidis and that in some species, galactosyltransferase in the luminal fluid is unlikely to have any function. The results are also discussed with respect to the possible function of galactosyltransferase, pyrophosphatase and phosphatase in epididymal luminal plasma and rete testis fluid.     

New approaches for field studies of mammals: experiences with marine mammals

The constraints imposed by studying a mammal in an aquatic environment and by the nerd to use benign methods have made it necessary to develop novel approaches in order to investigate the biology of marine mammals. The approaches have been made possible by recent technological advances and by the willingness of granting agencies to fund expensive, high–risk projects in marine science.
We review new trchniques which have been developed for estimating the population size of marine mammals, for investigating the relationsip between individuals and populations, for studying the behaviour and energetics of animals in the open sea, and for the management of small and endangered populations. We also indicate how these techniques may be applied to a variety of terrestrial mammals.     

Non-apoptotic phosphatidylserine externalization induced by engagement of glycosylphosphatidylinositol-anchored proteins

The exposure of phosphatidylserine (PS) on the cell surface is a general marker of apoptotic cells. Non-apoptotic PS externalization is induced by several activation stimuli, including engagement of immunoreceptors. Immune cells can also be activated by aggregation of glycosylphosphatidylinositol-anchored proteins (GPI-APs). However, it is unknown whether cell triggering through these proteins, lacking transmembrane and cytoplasmic domains, also leads to PS externalization. Here we show that engagement of GPI-APs in rodent mast cells induces a rapid and reversible externalization of PS by a non-apoptotic mechanism. PS externalization triggered by GPI-AP-specific monoclonal antibodies was dependent on the activity of H(+)-ATP synthase and several other enzymes involved in mast cell signaling but was independent of cell degranulation, free cytoplasmic calcium up-regulation, and a decrease in lipid packing as determined by merocyanine 540 binding. Surprisingly, disruption of actin cytoskeleton by latrunculin B or plasma membrane integrity by methyl-beta-cyclodextrin had opposite effects on PS externalization triggered through GPI-AP or the high affinity IgE receptor. We further show that PS externalization mediated by GPI-APs was also observed in some other cells, and its extent varied with antibodies used. Interestingly, effects of different antibodies on PS externalization were additive, indicating that independent stimuli converge onto a signaling pathways leading to PS externalization. Our findings identify the cell surface PS exposure induced through GPI-AP as a distinct mechanism of cell signaling. Such a mechanism could contribute to inside-out signaling in response to pathogens and other external activators and/or to initiation of other functions associated with PS externalization.     

Opportunities for reintroducing British mammals

There is good evidence that die three rarer British predatory mammals, the Polecat, Pine Marten and Wild Cat, owe their present restricted distribution to intense human persecution. Since this persecution has now diminished considerably, it would be feasible to attempt to reintroduce them to parts of Britain where they were exterminated last century. More important, however, would be attempts to reintroduce mammals which became totally extinct in historic times, including Aurochs, Boar, Brown Bear, Beaver and Wolf. The last of these survived the longest, and is a prime candidate for consideration. There are good ecological reasons for attempting to reintroduce it to Rhum, where the large herd of Red Deer has to be culled by one sixth each year in an attempt to reduce overgrazing and starvation. Evidence of studies in North America suggests that Wolf predation would be a far more efficient way of controlling the Red Deer; and it would be more in keeping with the management objectives of a National Nature Reserve. Similarly an attempt should be made to use genetically 'reconstituted' Tarpan and Aurochs to diversify the grazing, rather than domestic ponies and cattle.     

Molecular evidence for the early divergence of placental mammals

Paleontological and molecular data suggest quite different patterns for the early evolution of placental mammals. Paleontological evidence indicates a radiation, with most of the extant orders diverging at approximately the same time, close to the Cretaceous-Tertiary boundary, 65 Myr ago. Molecular evidence suggests a branching pattern of evolution that started much earlier. Resolving this discrepancy requires a consideration of the assumptions that underlie both approaches. It is argued here that the pattern indicated by the molecular approach is the most likely to be correct. If it is correct then either: 1) A diversity of placental mammals remains to be sampled from the Cretaceous, or 2) The placental orders diverged phylogenetically long before they diversified morphologically, implying a decoupling of the evolutionary processes associated with speciation and adaptation. The adaptive diversification of placental mammals may have required the demise of the dinosaurs at the end of the Cretaceous, but it occurred in lineages that had a long prior history of independent existence. 1999.