Identification of an unknown 31P nuclear magnetic resonance from dystrophic chicken as L-serine ethanolamine phosphodiester

Corneal absorption of glycerylphosphorylcholine

This study documents the absorption of glycerylphosphorylcholine (GPC) into corneas ex vivo. Corneas in quadruplicate were incubated in preservation medium containing 30 mM GPC, which is used as a reference marker. The GPC reference marker is used to calibrate ³¹P nuclear magnetic resonance (NMR) spectral chemical-shift positions for identification of phosphatic metabolites and to calculate intracorneal pH in intact tissues ex vivo. Following baseline NMR ex vivo analysis, corneas were stored in eye bank chambers in preservation medium containing 30 mM GPC at 4 °C overnight for 8 h. After returning to room temperature, NMR analysis was repeated on the same corneas in fresh GPC-free preservation medium. NMR analysis also was performed on the 30 mM GPC preservation medium alone from the eye bank chambers for detection of the GPC signal. The elevated GPC signal unexpectedly persisted in corneas incubated at 4 °C overnight even though GPC was not present in the fresh GPC-free preservation medium. In fact, the concentration of GPC in the intact cornea was many times higher than that found in the cornea endogenously. The levels of phosphatic metabolites and the energy modulus, after subtracting the spectral contribution of the 30 mM exogenous GPC, as well as the intracorneal pH remained unchanged from pre-refrigeration analyses. Corneas also retained transparency through the time-course of this study irrespective of temperature or change in temperature. The GPC signal in the NMR analysis of the preservation medium from the eye bank chambers was nearly undetectable. GPC was unexpectedly absorbed into the corneal tissue without detectable metabolic or physical toxicity. The intracorneal uptake of GPC at reduced temperatures parallels the increase in GPC that occurs naturally in muscle tissue in animals during wintering periods and the very high concentration of GPC in sperm, a cryogenically compatible cell, suggestive of a potential role for GPC in cryopreservation.

N-acyl-O-phosphocholineserines: structures of a novel class of lipids that are biomarkers for Niemann-Pick C1 disease

Niemann-Pick disease type C1 (NPC1) is a fatal, neurodegenerative, cholesterol storage disorder. With new therapeutics in clinical trials, there is an urgency to improve diagnostics and monitor therapeutic efficacy with biomarkers. In this study, we sought to define the structure of an unknown lipid biomarker for NPC1 with [M + H]⁺ ion at m/z 509.3351, previously designated as lysoSM-509. The structure of N-palmitoyl-O-phosphocholineserine (PPCS) was proposed for the lipid biomarker based on the results from mass spectrometric analyses and chemical derivatizations. As no commercial standard is available, authentic PPCS was chemically synthesized, and the structure was confirmed by comparison of endogenous and synthetic compounds as well as their derivatives using liquid chromatography-tandem mass spectrometry (LC-MS/MS). PPCS is the most abundant species among N-acyl-O-phosphocholineserines (APCS), a class of lipids that have not been previously detected in biological samples. Further analysis demonstrated that all APCS species with acyl groups ranging from C14 to C24 were elevated in NPC1 plasma. PPCS is also elevated in both central and peripheral tissues of the NPC1 cat model. Identification of APCS structures provide an opportunity for broader exploration of the roles of these novel lipids in NPC1 disease pathology and diagnosis.

Effect of creatine monohydrate in improving cellular energetics and muscle strength in ambulatory Duchenne muscular dystrophy patients: a randomized, placebo-controlled 31P MRS study

Randomized, placebo-controlled single blinded study was carried out to evaluate the effect of oral creatine supplementation on cellular energetics, manual muscle test (MMT) score and functional status in steroid-naive, ambulatory boys suffering with Duchenne muscular dystrophy (DMD; n=33). Eighteen patients received creatine monohydrate (Cr; 5 g/day for 8 weeks), while 15 received placebo (500 mg of vitamin C). Phosphorus metabolite ratios were determined from the right calf muscle of patients using phosphorus magnetic resonance spectroscopy ((31)P MRS) both prior to (baseline) and after supplementation of Cr or placebo. In addition, metabolite ratios were determined in normal calf muscle of age and sex matched controls (n=8). Significant differences in several metabolite ratios were observed between controls and DMD patients indicating a lower energy state in these patients. Analysis using analysis of covariance adjusted for age and stature showed that the mean phosphocreatine (PCr)/inorganic phosphate (Pi) ratio in patients treated with Cr (4.7; 95% CI; 3.9-5.6) was significantly higher (P=.03) compared to the placebo group (3.3; 95% CI; 2.5-4.2). The mean percentage increase in PCr/Pi ratio was also more in patients <7 years of age compared to older patients after Cr supplementation indicating variation in therapeutic effect with the age. In the placebo group, significant reduction in PCr/Pi (P=.0009), PCr/t-ATP (P=.05) and an increase in phosphodiester (PDE)/PCr ratios was observed after supplementation. Further, in the placebo group, patients <7 years showed reduction of PCr/t-ATP and Pi/t-ATP compared to older patients (>7 years), after supplementation. These results imply that the significant difference observed in PCr/Pi ratio between the Cr and the placebo groups after supplementation may be attributed to a decrease of PCr in the placebo group and an increase in PCr in the Cr group. Changes in MMT score between the two groups was significant (P=.04); however, no change in functional scale (P=.19) was observed. Parents reported subjective improvement on Cr supplementation versus worsening in placebo (P=.02). Our results indicated that Cr was well tolerated and oral Cr significantly improved the muscle PCr/Pi ratio and preserved the muscle strength in short term. However, this study provides no evidence that creatine will prove beneficial after long-term treatment, or have any positive effect on patient lifespan.

In vivo 31P MRS study of skeletal muscle metabolism in patients with postpolio residual paralysis

The muscle metabolism of at-rest patients with varying degrees of postpolio residual paralysis (PPRP) was studied and compared with that of controls using in vivo phosphorus magnetic resonance spectroscopy. The phosphocreatine (PCr)/inorganic phosphate (Pi) and PCr/adenosine triphosphate ratios were lower in patients than in controls. Reduction in PCr/Pi suggests abnormalities in oxidative phosphorylation. A significant increase was observed in the phosphomonoester/PCr ratio in patients, indicating the accumulation of intermediary compounds of the glycolytic pathway. Furthermore, the phosphodiester/PCr ratio was also significantly increased in patients. In general, the observed changes in metabolite ratios were found to be related to the degree of residual paralysis, suggesting that metabolic changes are secondary to chronic neurogenic processes. These metabolic alterations appear to be the possible cause of energy deficit and underlying muscle fatigue in PPRP patients. The present results provide an insight into the metabolic impairment and degree of muscle damage in patients with PPRP.

Distribution and potential role of cytosolic water-soluble phosphodiesters in fish

The distribution of water-soluble phosphodiesters (WSPDEs) visible by nuclear magnetic resonance (NMR) in some intact tissues of rainbow trout (Oncorhynchus mykiss walbaum) and in perchloric extracts after partial purification was examined by (31)P NMR spectroscopy. The compounds of interest were serine ethanolamine phosphate (SEP), threonine ethanolamine phosphate (TEP), glycerophosphorylcholine (GPC), and glycerophosphorylethanolamine (GPE). TEP and SEP were mostly accumulated in the heart and less accumulated in the kidney of intact trout. After the extraction procedure, two additional minor resonances were visible and identified as GPC and GPE. The liver of trout contained large amounts of GPE. Similar investigations were conducted by (31)P NMR on hearts and kidneys of two elasmobranchs (Scyliorhinus canicula, Raja clavata) and four teleosts (Anguilla anguilla, Sparus auratus, Dicentrarchus labrax, Scophtlhalmus maximus); comparison with the trout data showed striking interspecies differences in the identity of WSPDEs. All teleosts, except eel and turbot, accumulated predominantly TEP. However, in elasmobranchs, first GPC and then GPE were the major compounds. Whatever the studied species, the relative abundances in the heart and kidney were similar. In the last two decades, two hypotheses were proposed to explain the occurrence of high levels of cytosoluble phosphodiesters: these compounds may constitute an index of phospholipid catabolism or a protective mechanism through which phospholipid levels are kept high. To test them and elucidate the role of these compounds in membrane phospholipid regulation in fish, we investigated the effects of two physiological stresses, that is, seawater adaptation and induced myocardial ischemia, on trout cytosolic phosphodiester levels. A 32.5-min ischemic stress caused no effect on SEP and TEP levels. On the contrary, significant osmotic stress induced changes in the PDEs levels: 2 d after transfer from freshwater to seawater or from seawater to freshwater, both tissues displayed a transient decrease of TEP; however, a 2-d stay in seawater after transfer from freshwater caused a rise in SEP concentration, whereas a 2-d stay in freshwater after transfer decreased SEP level. In conclusion, our experiments suggest a relationship between the high levels of cytosoluble phosphodiesters observed in some fish tissues and resistance to stress.

A 31P nuclear magnetic resonance study of the hydrothermal vent tube worm Riftia pachyptila

31P nuclear magnetic resonance (NMR) was used to study the major phosphorylated compounds visible in perchloric extracts of three body regions of the vestimentiferan worm Riftia pachyptila: winged vestimentum, trunk and segmented posterior opisthosome. Two phosphagens (PGs) were present in vestimentum and opisthosome. The major resonance corresponded to those of phosphoarginine and phosphotaurocyamine, which cannot be discriminated on 31P NMR spectra. We have identified four distinct phosphodiesters (PDEs) in these tissues: glycerophosphorylethanolamine (GPE), serine ethanolamine phosphodiester (SEP), glycero-phosphorylcholine (GPC) and threonine ethanolamine phosphodiester (TEP). Three phosphonates or derivates (PAs) were observed in the three body regions. The minor one was identified as 2-aminoethyl phosphonate (2-AEP). The phosphorus profile of the trunk was appreciably different: one additional resonance in the PDE region and only one phosphagen peak were observed.

31P-NMR determinations of cytosolic phosphodiesters in turtle hearts

As part of our ongoing research on cardiac hypoxia tolerance we have conducted 31P nuclear magnetic resonance (NMR) studies of isolated, perfused, working hearts from freshwater turtles, animals that are well known for their ability to tolerate prolonged periods of anoxia. A striking feature of turtle heart spectra is an extremely high concentration of NMR visible phosphodiesters (PDEs). Cardiac spectra from mammals, on the other hand, typically exhibit only a small resonance in the PDE region. Our aim in this study was to compare myocardial PDE profiles between the highly hypoxia tolerant western painted turtle (Chrysemys picta bellii) and the relatively hypoxia sensitive softshelled turtle (Trionyx spinifer) in order to begin to rest the hypothesis that high constitutive levels of cytosolic PDEs may play a role in conferring hypoxia and ischemia tolerance on the myocardium. We also collected 31P-NMR spectra of PCA extracts of tissue from these species and from Kemp's ridley sea turtles (Lepidochelys kempi), as well as spectra from isolated hearts and PCA extracts of red-eared sliders (Trachemys [formerly Pseudemys] scripta]). Total NMR visible phosphodiesters make up 24 +/- 8.6% of the total NMR visible phosphorus in Chrysemys hearts, 20.7 +/- 5.9% in Trachemys hearts, but only 12.2 +/- 5.1% in Trionyx hearts (P < 0.05). We have identified three distinct PDEs in turtle hearts: glycerophosphorylcholine (GPC); glycerophosphorylethanolamine (GPE); and serine ethanolamine phosphodiester (SEP). SEP is the dominant compound in Chrysemys and Trachemys (79.3 +/- 10.2% and 84.7 +/- 3.7% of total PDE, respectively), while GPC is most abundant in Trionyx (74.0 +/- 4.3% of total PDE) and Lepidochelys (not quantitated). The function of this class of compounds is unclear but it has been suggested that cytosolic PDEs may function as lysophospholipase inhibitors, a role that would decrease the rate of membrane phospholipid turnover. Our comparative data suggest that cytosolic PDEs could play a role in phospholipid sparing during anoxic or ischemic stress in turtles but a direct test of this hypothesis awaits future experimentation.

The effect of prolonged anoxia at 3 degrees C on tissue high energy phosphates and phosphodiesters in turtles: a 31P-NMR study

Selected tissues (skeletal muscle, heart ventrical, and liver), sampled from turtles (Chrysemys picta bellii) at 3 degrees C either under normoxic conditions or after 12 weeks of anoxic submergence were quantitatively analysed for intracellular pH and phosphorus metabolites using 31P-NMR. Plasma was tested for osmolality and for the concentrations of lactate, calcium, and magnesium to confirm anoxic stress. We hypothesized that, in the anoxic animals, tissue ATP levels would be maintained and that the increased osmolality of the body fluids of anoxic turtles would be accounted for by a corresponding increase in the concentrations of phosphodiesters. The responses observed differed among the three tissues. In muscle, ATP was unchanged by anoxia but phosphocreatine was reduced by 80%; in heart, both ATP and phosphocreatine fell by 35-40%. The reduction in phosphocreatine in heart tissue at 3 degrees C was similar to that observed in isolated, perfused working hearts from turtles maintained at 20 degrees C but no decrease in ATP occurred in the latter tissues. In liver, although analyses of several specimens were confounded by line-broadening, neither ATP nor phosphocreatine was detectable in anoxic samples. Phosphosdiesters were detected in amounts sufficient to account for 30% of normoxic cell osmotic concentration in heart and 11% and 12% in liver and muscle, respectively. The phosphodiester levels did not change in anoxia. Heart ventricular phosphodiester levels in turtles at 3 degrees C were significantly higher than those determined for whole hearts from turtles at 20 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycerol phosphorylcholine (GPC) and serine ethanolamine phosphodiester (SEP): evolutionary mirrored metabolites and their potential metabolic roles

Water-soluble phosphodiesters (WSPDE) are a prominent feature of many 31P-NMR spectra; however, their role has remained somewhat of a mystery. What has been missed in almost all previous studies is the fact that two classes of WSPDE exist in vertebrates: those in mammals and those in the other (reptile-avian) line. The first is represented by glycerol phosphorylcholine and the second by serine ethanolamine phosphodiester. A further examination of the literature suggests a common role for all WSPDE as lysophospholipase inhibitors and therefore net sparers of phospholipids by decreasing phospholipid metabolic throughput.

An in vivo phosphorus nuclear magnetic resonance study of the variations with age in the phosphodiester content of human muscle

Human gastrocnemius and slow twitch muscles contain phosphodiesters that may be detected in vivo by phosphorus nuclear magnetic resonance (NMR). This work represents a study of 354 spectra obtained from healthy subjects of various ages and from patients with peripheral vascular disease. The analysis of the data indicate a correlation between the concentration of phosphodiesters and age. By comparing the data obtained with healthy subjects and patients it is concluded that the increase in phosphodiesters is not due to disease, but to ageing itself. The significance of this increase is discussed.

Mobile phospholipid signals in NMR spectra of cultured human adenocarcinoma cells

An intense peak is visible in the phosphodiester region of the 31P NMR spectra of human colon adenocarcinoma HCT-8R cells. A signal at the same frequency, partially deriving from mobile phospholipids, is also observable in the spectra of total membranes and of perchloric acid extracts prepared from the same cells. The phosphodiester signals of glycerophosphorylcholine, glycerophosphorylethanolamine, and glycerophosphorylserine resonate at lower fields with respect to this broader peak and appear only in aged samples. Enzymatic treatments of the extracts would indicate that ribonucleic acid also contributes to the broad phosphodiester signal. Moreover, 1H NMR spectra show the presence of lipid structures, characterized by high mobility, in the cell and in the membrane samples, similar to what was already observed in other tumor cells and tissues.

In vivo proton spin-lattice relaxation times of normal and dystrophic muscles

In vivo spin-lattice relaxation times (T1) of water and lipid protons were measured in normal and dystrophic chicken pectoralis muscles at different ages. Values were obtained with a surface coil used as both a receiver and a transmitter. A 2 theta-T1-theta-Acquisition sequence was used for these measurements. Accuracy was verified with an inversion-recovery method using a slotted tube resonator as the transmitter and a surface coil as the receiver. It was observed that the T1 values of water protons in normal muscles decrease with age, the T1 values of water protons do not change with age in dystrophic muscles, and the T1 values of lipid protons increase with age in normal and dystrophic muscles. These results indicate a failure of the normal maturation of dystrophic muscles.

Phosphorus-31 nuclear magnetic resonance study of post mortem catabolism and intracellular pH in intact excised rabbit muscle

Phosphorus-31 nuclear magnetic resonance has been used to study the post mortem catabolism of high-energy phosphate compounds and the associated intracellular pH variation in pure fast- and slow-twitch rabbit muscles and in rabbit muscle with mixed fiber types. Comparative results from pure fiber types are reported for the first time. Large amounts of glycerophosphorylcholine (14.1 mumol/g fresh tissue) are found in the internal conoidal bundle (ICB), a pure oxidative slow twitch muscle, whereas the m. psoas major (PM), a pure glycolytic fast twitch muscle and the m. gastrocnemius caput medialis (GCM), with mixed fiber types, are devoid of the same metabolite. The total content of phosphorylated metabolites is constant among the three muscle types. The time-dependent post mortem changes in phosphorylated metabolites display the expected rapid drop in phosphocreatine and a simultaneous increase in intracellular inorganic phosphate. However, the ATP level remains constant during more than 2 h. Rate constants for metabolite breakdown and apparent ATPase activity have been determined. The comparative kinetics of intracellular acidosis at 25 degrees C yield rates of 3.3 X 10(-3) pH unit/min for PM, 2.7 X 10(-3) pH unit/min for GCM and 3.0 X 10(-3) pH unit/min for ICB. Initial intracellular pH values are 7.07, 7.20 and 7.02, respectively. Upon aging, the heterogeneity of the Pi signal reflects the existence of cellular compartments with different internal pH. The results suggest that the more intense low-pH Pi signal arises from the sarcoplasmic reticulum while the less intense resonance would reflect the sarcoplasmic higher pH. The temperature effect on post mortem catabolism in the 15-25 degrees C range has been documented. As expected, phosphocreatine and ATP breakdown increase with temperature but at a higher rate for slow-twitch ICB than for fast-twitch PM.

31P surface-coil NMR analysis of metabolic status in KHJJ tumors

Noninvasive monitoring of the effects of treatment on cancer tissue is fundamental to the development of rational radioimmunotherapy (RIT) schemes employing radiolabeled monoclonal antibodies or antibody fragments specific for human cancer. Recent advances in nuclear magnetic resonance spectroscopy make it an attractive candidate for sequential, topical monitoring by 31P NMR of metabolic events during RIT. Preliminary to this effort, we examined the metabolic competence of the well-characterized, murine KHJJ tumor in situ in BALB/c mice using 31P surface-coil NMR. The ATP/Pi ratio in tumor volumes ranging from 100 to 800 mm3 showed that tumors over this range of sizes were able to maintain high levels of ATP relative to Pi. As the tumor volume increased above 1 cm3, ATP/Pi levels indicated poor metabolic competence. This lack of metabolic competence was correlated with histological evidence of tissue necrosis and vascular disintegration. The T1 values of assigned phosphorus metabolites were established. Intracellular pH, as determined from the chemical shift of Pi, was found to vary in these tumors from 7.1 in rapidly metabolizing tissue to 6.6 in necrotic tumors.

[Study of in vivo cellular metabolism by phosphorus 31 nuclear magnetic resonance]

Phosphorus-31 nuclear magnetic resonance spectroscopy has been recently increasingly used to study cellular metabolism in a manner respecting the cell integrity. Intrinsic advantages of the phosphorus nucleus for in vivo NMR studies are discussed in this review together with some selected applications. A particular emphasis is layed on metabolite identification and quantitation (relative and absolute concentrations), the measurement of intracellular pH and the problem of cellular compartmentation. The determination of metabolite fluxes under normal and abnormal biological and physiological conditions, and the in vivo direct measurement by saturation transfer techniques of kinetic parameters for enzymatic reactions at equilibrium, are illustrated by several examples taken from the available literature and work carried out in this laboratory. Whenever possible, and appropriate, the NMR approach has been compared with other more classical techniques of investigation. The future and the potentialities of phosphorus-31 NMR study of intact biological systems, the clinical applications and the foreseeable interfacing with imaging techniques are evaluated. The concept of "functional imaging" versus "anatomic imaging" is proposed to illustrate the impact of this new technology in the understanding of cellular mechanisms, not only in the intact cell but also in whole tissues or organs after excision or in living animals and human.

Nuclear magnetic resonance study of muscle water protons in muscular dystrophy of chickens

Using the pulsed nuclear magnetic resonance (NMR) spectroscopy, the spin-lattice (T1) and the spin-spin (T2) relaxations times of water protons from samples of pectoralis major muscles of normal (line 412) and homozygous dystrophic (line 413) chickens were measured. Both the T1 and T2 were significantly increased (P less than 0.05) in the dystrophic muscles. The mean values of the relaxation times are given +/- S.D. The T1 values were 654 +/- 22 msec in normal and 692 +/- 41 msec in dystrophic muscles. The T2 values for normal and dystrophic muscles were 39 +/- 4 msec and 52 +/- 7 msec, respectively. Although the water content of dystrophic muscles (78.9 +/- 0.6%) determined by gravimetric methods was significantly higher than normal muscles (74.9 +/- 1.1%), this difference in tissue hydration could not explain quantitatively the increase of T1 and T2 values in the dystrophic muscles. The results of the measurements of the relaxation times seem to suggest that there are changes in the composition and/or conformational state of the proteins.

Mechanical relaxation rate and metabolism studied in fatiguing muscle by phosphorus nuclear magnetic resonance

1. We have used phosphorus nuclear magnetic resonance (31P NMR) to study muscular fatigue in anaerobic amphibian muscle. In this paper the biochemical and energetic changes that result from a series of tetani are related to the decrease in rate constant (1/tau) for the final, exponential, phase of relaxation. 2. Using 31P NMR we have measured the concentrations of phosphocreatine (PCr), inorganic phosphate (Pi) and ATP as well as the internal pH. From our measurements we have calculated [creatine], [free ADP], the free-energy change (more precisely, the affinity A = -dG/d xi) for ATP hydrolysis and the rates of lactic acid production and of ATP hydrolysis. 3. We have found that 1/tau, the rate constant of relaxation, is correlated with each of the following, independently of the pattern of stimulation: isometric force production, all of the measured or calculated metabolite levels, pH and dG/d xi. 4. There is a clear dependence upon the pattern of stimulation of the relation between 1/tau and each of the following: total duration of the experiment, number of contractions, rate of lactic acid production and rate of ATP hydrolysis. 5. The rate of relaxation is linearly related to [PCr], [creatine], [Pi] and dG/d xi. It is nonlinearly related to isometric force, [ATP], [H+] and rate of ATP hydrolysis. 6. We conclude that the change in 1/tau, like that of isometric force, depends upon metabolic factors, and not upon any independent changes in the activation or deactivation of contraction. We suggest that 1/tau may depend upon the free-energy change for ATP hydrolysis which in turn may be related to the rate of Ca2+ uptake into the sarcoplasmic reticulum.

Phosphodiesters in muscular dystrophies

Phosphorus nuclear magnetic resonance studies of various normal and dystrophic muscles have demonstrated differences in phosphodiester contents. Dystrophic chicken pectoralis muscle contains elevated levels of the diester SEP, and Duchenne dystrophic human leg muscle lacks the diester GPC. It is shown that SEP may be characteristic of slow fiber types, indicating imcomplete maturation of the dystrophic pectoralis muscle. The activities of SEP-metabolizing enzymes have been determined in microsomes of pectoralis muscle, kidney, and intestinal mucosa of normal and dystrophic chickens. There was little difference between normal and dystrophic values of SEP phosphodiesterase and SEP synthase in kidney and mucosa. Dystrophic pectoralis muscle microsomes possessed significantly elevated levels of both enzymes. The increase in SEP synthase activity of dystrophic muscle may explain the increase in the SEP level if the phosphodiesterase is regulated in vivo. It is shown that human muscle disease processes may raise, leave unchanged, or lower GPC levels. In certain diseases when GPC is markedly elevated, such as Werdnig-Hoffmann, or depressed, such as Duchenne muscular dystrophy, it may serve as a marker.

Muscular fatigue investigated by phosphorus nuclear magnetic resonance

Muscular fatigue has been studied using 31PNMR to measure the levels and rates of utilisation of several key metabolites and the free-energy change for ATP hydrolysis. Force development is closely correlated with metabolite levels and is proportional to the rate at which ATP is hydrolysed.

Serine ethanolamine phosphodiester: a major component in chicken semen

The phosphodiester, serine ethanolamine phosphodiester, has been found by 31P NMR to be present in chicken semen and the structure was confirmed by chromatographic and spectroscopic techniques. The presence of serine ethanolamine phosphodiester in chicken semen completes the analogy between the distribution of this compound in the tissues of the chicken and that of glycerol 3-phosphorylcholine in mammals.