The inhibition of neuraminidase and antiviral action

The convergent evolution of influenza A virus: Implications, therapeutic strategies and what we need to know

Influenza virus infection, more commonly known as the 'cold flu', is an etiological agent that gives rise to recurrent annual flu and many pandemics. Dated back to the 1918- Spanish Flu, the influenza infection has caused the loss of many human lives and significantly impacted the economy and daily lives. Influenza virus can be classified into four different genera: influenza A-D, with the former two, influenza A and B, relevant to humans. The capacity of antigenic drift and shift in Influenza A has given rise to many novel variants, rendering vaccines and antiviral therapies useless. In light of the emergence of a novel betacoronavirus, the SARS-CoV-2, unravelling the underpinning mechanisms that support the recurrent influenza epidemics and pandemics is essential. Given the symptom similarities between influenza and covid infection, it is crucial to reiterate what we know about the influenza infection. This review aims to describe the origin and evolution of influenza infection. Apart from that, the risk factors entail the implication of co-infections, especially regarding the COVID-19 pandemic is further discussed. In addition, antiviral strategies, including the potential of drug repositioning, are discussed in this context. The diagnostic approach is also critically discussed in an effort to understand better and prepare for upcoming variants and potential influenza pandemics in the future. Lastly, this review encapsulates the challenges in curbing the influenza spread and provides insights for future directions in influenza management.

Antiviral Potential of Natural Resources against Influenza Virus Infections

Influenza is a severe contagious disease caused by influenza A and B viruses. The WHO estimates that annual outbreaks lead to 3-5 million severe infections of which approximately 10% lead to the death of the patient. While vaccination is the cornerstone of prevention, antiviral drugs represent the most important treatment option of acute infections. Only two classes of drugs are currently approved for the treatment of influenza in numerous countries: M2 channel blockers and neuraminidase inhibitors. In some countries, additional compounds such as the recently developed cap-dependent endonuclease inhibitor baloxavir marboxil or the polymerase inhibitor favipiravir are available. However, many of these compounds suffer from poor efficacy, if not applied early after infection. Furthermore, many influenza strains have developed resistances and lost susceptibility to these compounds. As a result, there is an urgent need to develop new anti-influenza drugs against a broad spectrum of subtypes. Natural products have made an important contribution to the development of new lead structures, particularly in the field of infectious diseases. Therefore, this article aims to review the research on the identification of novel lead structures isolated from natural resources suitable to treat influenza infections.

Influenza A Virus Neuraminidase Inhibitors

Depending on the strain, influenza A virus causes animal, zoonotic, pandemic, or seasonal influenza with varying degrees of severity. Two surface glycoprotein spikes, hemagglutinin (HA) and neuraminidase (NA), are the most important influenza A virus antigens. NA plays an important role in the propagation of influenza virus by removing terminal sialic acid from sialyl decoy receptors and thereby facilitating the release of viruses from traps such as in mucus and on infected cells. Some NA inhibitors have become widely used drugs for treatment of influenza. However, attempts to develop effective and safe NA inhibitors that can be used for treatment of anti-NA drugs-resistant influenza viruses have continued. In this chapter, we describe the following updates on influenza A NA inhibitor development: (i) N-acetylneuraminic acid (Neu5Ac)-based derivatives, (ii) covalent NA inhibitors, (iii) sulfo-sialic acid analogs, (iv) N-acetyl-6-sulfo-β-D-glucosaminide-based inhibitors, (v) inhibitors targeting the 150-loop of group 1 NAs, (vi) conjugation inhibitors, (vii) acylhydrazone derivatives, (viii) monoclonal antibodies, (ix) PVP-I, and (x) natural products. Finally, we provide future perspectives on the next-generation anti-NA drugs.

Sialidase and Sialyltransferase Inhibitors: Targeting Pathogenicity and Disease

Sialidases (SAs) and sialyltransferases (STs), the enzymes responsible for removing and adding sialic acid to other glycans, play essential roles in viruses, bacteria, parasites, and humans. Sialic acid is often the terminal sugar on glycans protruding from the cell surface in humans and is an important component for recognition and cell function. Pathogens have evolved to exploit this and use sialic acid to either “cloak” themselves, ensuring they remain undetected, or as a mechanism to enable release of virus progeny. The development of inhibitors against SAs and STs therefore provides the opportunity to target a range of diseases. Inhibitors targeting viral, bacterial, or parasitic enzymes can directly target their pathogenicity in humans. Excellent examples of this can be found with the anti-influenza drugs Zanamivir (Relenza™, GlaxoSmithKline) and Oseltamivir (Tamiflu™, Roche and Gilead), which have been used in the clinic for over two decades. However, the development of resistance against these drugs means there is an ongoing need for novel potent and specific inhibitors. Humans possess 20 STs and four SAs that play essential roles in cellular function, but have also been implicated in cancer progression, as glycans on many cancer cells are found to be hyper-sialylated. Whilst much remains unknown about how STs function in relation to disease, it is clear that specific inhibitors of them can serve both as tools to gain a better understanding of their activity and form the basis for development of anti-cancer drugs. Here we review the recent developments in the design of SA and ST inhibitors against pathogens and humans.

Synthesis, inhibitory activity and oral dosing formulation of AV5124, the structural analogue of influenza virus endonuclease inhibitor baloxavir

BACKGROUND

The development and clinical implementation of the cap-dependent endonuclease (CEN) inhibitor baloxavir marboxil was a breakthrough in influenza therapy, but it was associated with the emergence of drug-resistant variants.

OBJECTIVES

To design and synthesize structural analogues of CEN inhibitors and evaluate their safety, pharmacokinetics and antiviral potency in vitro and in vivo.

METHODS

The drug candidate AV5124 and its active metabolite AV5116 were synthesized based on pharmacophore modelling. Stability in plasma and microsomes, plasma protein binding, cytotoxicity and antiviral activities were assessed in vitro. Pharmacokinetics after IV or oral administration were analysed in CD-1 mice. Acute toxicity and protective efficacy against lethal A(H1N1)pdm09 influenza virus challenge were examined in BALB/c mice.

RESULTS

Pharmacophore model-assisted, 3D molecular docking predicted key supramolecular interactions of the metal-binding group and bulky hydrophobic group of AV5116 with the CEN binding site (Protein Data Bank code: 6FS6) that are essential for high antiviral activity. AV5116 inhibited influenza virus polymerase complexes in cell-free assays and replication of oseltamivir-susceptible and -resistant influenza A and B viruses at nanomolar concentrations. Notably, AV5116 was equipotent or more potent than baloxavir acid (BXA) against WT (I38-WT) viruses and viruses with reduced BXA susceptibility carrying an I38T polymerase acidic (PA) substitution. AV5116 exhibited low cytotoxicity in Madin-Darby canine kidney cells and lacked mitochondrial toxicity, resulting in favourable selective indices. Treatment with 20 or 50 mg/kg AV5124 prevented death in 60% and 100% of animals, respectively.

CONCLUSIONS

Overall, AV5124 and A5116 are promising inhibitors of the influenza virus CEN and warrant further development as potent anti-influenza agents.

Isocorilagin, isolated from Canarium album (Lour.) Raeusch, as a potent neuraminidase inhibitor against influenza A virus

Canarium album (Lour.) Raeusch (C. album) as a normally medicinal and edible plant has been used widely in Asian countries and is considered a source of phytochemicals that are beneficial to human health. Here, we showed at the first time isocorilagin, a polyphenolic compound isolated from C. album, displayed antiviral activity against diverse strains of influenza A virus (IAV), including A/Puerto Rico/8/34 (H1N1), A/Aichi/2/68 (H3N2) and NA-H274Y (H1N1) with IC₅₀ value of 9.19 ± 1.99, 23.72 ± 2.51 and 4.64 ± 3.01 μM, respectively. Further mechanistic studies revealed that it clearly inhibited neuraminidase activity of IAV and directly influenced the virus release. The molecular docking studies presented isocorilagin could bind to the highly conserved residues in the active sites of NA, implying that isocorilagin may be effective against various influenza strains and not susceptible to produce drug resistance. Taken together, the results strongly suggest that isocorilagin has potential to be an effective, safe and affordable neuraminidase inhibitor against a diverse panel of IAV strains. More importantly, our work expands the biological activities of C. album extracts and provide a new option for the development of anti-influenza drug.

Glycopeptides and -Mimetics to Detect, Monitor and Inhibit Bacterial and Viral Infections: Recent Advances and Perspectives

The initial contact of pathogens with host cells is usually mediated by their adhesion to glycan structures present on the cell surface in order to enable infection. Furthermore, glycans play important roles in the modulation of the host immune responses to infection. Understanding the carbohydrate-pathogen interactions are of importance for the development of novel and efficient strategies to either prevent, or interfere with pathogenic infection. Synthetic glycopeptides and mimetics thereof are capable of imitating the multivalent display of carbohydrates at the cell surface, which have become an important objective of research over the last decade. Glycopeptide based constructs may function as vaccines or anti-adhesive agents that interfere with the ability of pathogens to adhere to the host cell glycans and thus possess the potential to improve or replace treatments that suffer from resistance. Additionally, synthetic glycopeptides are used as tools for epitope mapping of antibodies directed against structures present on various pathogens and have become important to improve serodiagnostic methods and to develop novel epitope-based vaccines. This review will provide an overview of the most recent advances in the synthesis and application of glycopeptides and glycopeptide mimetics exhibiting a peptide-like backbone in glycobiology.

Antivirals against animal viruses

Antivirals are compounds used since the 1960s that can interfere with viral development. Some of these antivirals can be isolated from a variety of sources, such as animals, plants, bacteria or fungi, while others must be obtained by chemical synthesis, either designed or random. Antivirals display a variety of mechanisms of action, and while some of them enhance the animal immune system, others block a specific enzyme or a particular step in the viral replication cycle. As viruses are mandatory intracellular parasites that use the host's cellular machinery to survive and multiply, it is essential that antivirals do not harm the host. In addition, viruses are continually developing new antiviral resistant strains, due to their high mutation rate, which makes it mandatory to continually search for, or develop, new antiviral compounds. This review describes natural and synthetic antivirals in chronological order, with an emphasis on natural compounds, even when their mechanisms of action are not completely understood, that could serve as the basis for future development of novel and/or complementary antiviral treatments.

Drug resistance in influenza A virus: the epidemiology and management

Influenza A virus (IAV) is the sole cause of the unpredictable influenza pandemics and deadly zoonotic outbreaks and constitutes at least half of the cause of regular annual influenza epidemics in humans. Two classes of anti-IAV drugs, adamantanes and neuraminidase (NA) inhibitors (NAIs) targeting the viral components M2 ion channel and NA, respectively, have been approved to treat IAV infections. However, IAV rapidly acquired resistance against both classes of drugs by mutating these viral components. The adamantane-resistant IAV has established itself in nature, and a majority of the IAV subtypes, especially the most common H1N1 and H3N2, circulating globally are resistant to adamantanes. Consequently, adamantanes have become practically obsolete as anti-IAV drugs. Similarly, up to 100% of the globally circulating IAV H1N1 subtypes were resistant to oseltamivir, the most commonly used NAI, until 2009. However, the 2009 pandemic IAV H1N1 subtype, which was sensitive to NAIs and has now become one of the dominant seasonal influenza virus strains, has replaced the pre-2009 oseltamivir-resistant H1N1 variants. This review traces the epidemiology of both adamantane- and NAI-resistant IAV subtypes since the approval of these drugs and highlights the susceptibility status of currently circulating IAV subtypes to NAIs. Further, it provides an overview of currently and soon to be available control measures to manage current and emerging drug-resistant IAV. Finally, this review outlines the research directions that should be undertaken to manage the circulation of IAV in intermediate hosts and develop effective and alternative anti-IAV therapies.

Anti-influenza drugs: the development of sialidase inhibitors

Viruses, particularly those that are harmful to humans, are the 'silent terrorists' of the twenty-first century. Well over four million humans die per annum as a result of viral infections alone. The scourge of influenza virus has plagued mankind throughout the ages. The fact that new viral strains emerge on a regular basis, particularly out of Asia, establishes a continual socio-economic threat to mankind. The arrival of the highly pathogenic avian influenza H5N1 heightened the threat of a potential human pandemic to the point where many countries have put in place 'preparedness plans' to defend against such an outcome. The discovery of the first designer influenza virus sialidase inhibitor and anti-influenza drug Relenza, and subsequently Tamiflu, has now inspired a number of continuing efforts towards the discovery of next generation anti-influenza drugs. Such drugs may act as 'first-line-of-defence' against the spread of influenza infection and buy time for necessary vaccine development particularly in a human pandemic setting. Furthermore, the fact that influenza virus can develop resistance to therapeutics makes these continuing efforts extremely important. An overview of the role of the virus-associated glycoprotein sialidase (neuraminidase) and some of the most recent developments towards the discovery of anti-influenza drugs based on the inhibition of influenza virus sialidase is provided in this chapter.

Development of antivirals against influenza

Morbidity and mortality due to influenza virus infections remain a major problem throughout the world. Yearly, medical costs and loss of productivity resulting from influenza infection are estimated to be in the range of 12 dollars bn in the USA. The predicted increases in the elderly and immune-deficient populations will make influenza an even greater threat in the future. Despite the availability of vaccines, they have been least effective in these high-risk populations. Coupled with the requirement for routine revaccination, the need for effective antiviral agents is illustrated. The currently approved drugs, amantadine, rimantadine and ribavirin (in some countries), have limitations. They are only inhibitory against influenza A viruses, are prone to adverse reactions and quickly give rise to resistant virus. This review examines current drug therapies, antivirals in development and possible future opportunities for anti-influenza drugs.

Neuraminidase inhibitors as antivirals

The description of the three-dimensional structure of the influenza virus neuraminidase in 1983 opened a new phase in the search for inhibitors of the enzyme. Two compounds in late development (February 1999) have similar levels of potency against influenza A viruses, different routes of administration and, surprisingly, different resistance profiles both in vitro and in vivo.

Synthesis of glycophostones: cyclic phosphonate analogues of biologically relevant sugars

Analogues of L-fucose, N-acetyl-D-glucosamine, N-acetyl-D-mannosamine, and N-acetyl neuraminic acid in which the anomeric carbon atom was replaced by a phosphonyl group (phostones or cyclic phosphonates) were synthesized by stereocontrolled methods relying on the Abramov reaction.

'Flu' and structure-based drug design

The threat of a catastrophic outbreak of influenza is ever present. Vaccines are only partially effective and the two compounds, amantidine and rimantidine, used clinically against influenza A cause side-effects and rapid viral resistance. Recent advances bring hope that specific and potent drugs against influenza may soon be available in the clinic. These compounds were designed to inhibit influenza neuraminidase (NA), one of the viral coat glycoproteins, using the crystal structure of NA which was first published in 1983. In this review, the application of structure-based drug design approaches to the design of anti-influenza agents targeted at NA and haemagglutinin (HA), the other viral surface glycoprotein, is discussed.

Influenza virus neuraminidase: structure, antibodies, and inhibitors

The determination of the 3-dimensional structure of the influenza virus neuraminidase in 1983 has served as a platform for understanding interactions between antibodies and protein antigens, for investigating antigenic variation in influenza viruses, and for devising new inhibitors of the enzyme. That work is reviewed here, together with more recent developments that have resulted in one of the inhibitors entering clinical trials as an anti-influenza virus drug.

Group B streptococcal neuraminidase is actually a hyaluronidase

The extracellular group B streptococcal enzyme described in numerous reports as a neuraminidase is really a hyaluronidase. Over the past 25 years, the enzyme was routinely assayed with bovine submaxillary mucin as the substrate and by the thiobarbituric acid procedure to measure released sialic acid. Characterization of the actual compound released by the enzyme revealed it to be an alpha,beta-unsaturated derivative of hyalobiuronic acid that was derived from hyaluronic acid contaminating the mucin preparation. Previous reports describing an association of elevated levels of extracellular neuraminidase with virulent strains of group B streptococci must be reevaluated with the recognition that the enzyme is really a hyaluronidase.

The interaction of substrate-related ketals with bacterial and viral neuraminidases

Arthrobacter sialophilus neuraminidase catalyzes the hydration of 5-acetamido-2,6-anhydro-3,5-dideoxy-D-glycero-D-galacto-non-2-enonic acid (2,3-dehydro-AcNeu) with Km and kcat values of 8.9 X 10(-4) M and 6.40 X 10(-4) s-1, respectively. The methyl ester of 2,3-dehydro-AcNeu as well as 2,3-dehydro-4-epi-AcNeu are also hydrated by the enzyme. The product resulting from the enzymatic hydration of 2,3-dehydro-AcNeu is N-acetylneuraminic acid. A series of derivatives of 2,3-dehydro-AcNeu (K1, 1.60 X 10(-6) M) including 2,3-dehydro-4-epi-AcNeu (2.10 X 10(-4) M) and 2,3-dehydro-4-keto-AcNeu (K1 = 6.10 X 10(-5) M) were each competitive inhibitors of the enzyme. The methyl esters of these ketal derivatives were also competitive enzyme inhibitors. Dissociation constants for these ketals were determined independently by fluorescence enzyme titrations which gave values similar to those found kinetically. These six relatives of 2,3-dehydro-AcNeu were also competitive inhibitors for the influenza viral neuraminidases. For the viral neuraminidases, the dissociation constant for 2,3-dehydro-AcNeu and its methyl ester were 2.40 X 10(-6) and 1.17 X 10(-3) M, respectively. The interpretation placed upon the K1 values determined for these ketals against the Arthrobacter versus influenza neuraminidases is that the bacterial enzyme has a more flexible glycone binding site.

Heterogeneity of lysosomes originating from rat liver parenchymal cells. Metabolic relationship of subpopulations separated by density-gradient centrifugation

1. A crude lysosomal fraction obtained by differential centrifugation of a rat liver homogenate was subjected to zonal centrifugation in iso-osmotic self-generating gradients composed of modified colloidal silica (Percoll). Analysis of relevant marker-enzyme activities shows a continuous band of considerably purified lysosomal particles in the density range 1.04--1.11 g/ml. 2. A relationship between age and buoyant density of the parenchymal lysosomal subpopulations is indicated by the distribution of 125I-labelled asialoglycoproteins in the heterogeneous lysosomes during the catabolism of the glycoprotein. The labelled asialoglycoprotein first appeared in lysosomal particles of low density, which with time progressively acquired a higher density. Furthermore, 30 min after administration the 125I-labelled asialocaeruloplasmin recovered in the light lysosomes was less degraded than the material recovered in the heavy lysosomes. 3. A lysosomal enzyme (arylsulphatase) was found to possess considerably higher isoelectric points in the heavy lysosomes than in the light lysosomes, which is consistent with a relationship between age and density of the lysosomes.

Rat liver microsomal and lysosomal beta-glucuronidases differ in both carbohydrate and amino acid compositions

To investigate the chemical relationships between rat liver lysosomal and microsomal beta-D-glucuronidases (EC 3.2.1.31), which are essentially identical catalytically and in reactivity with antibody and similar in molecular weight, the two enzymes were isolated by procedures in which modifications of the proteins were avoided. The purified enzymes were found to differ in both sugar and amino acid compositions. The microsomal enzyme contained much more mannose and, in contrast to the lysosomal enzyme, contained sialic acid but no glucose. Moreover, although the amino acid compositions generally agreed closely, the microsomal enzyme contained much more serine and somewhat less arginine than the lysosomal form. These findings of specific differences in composition should have a bearing on the consideration of intracellular glycoprotein synthesis, translocation, and compartmentalization.

Sialoresponsin and an antiviral action of ascorbic acid

Several pathogens, both viral and bacterial, employ the enzyme neuraminidase (N-acetylneuraminate glycohydrolase, EC 3.2.1.1.8). The neuraminidase renders ineffective the hemagglutinin inhibitory mucins that confine the pathogens in a coating of host mucins. Sialoresponsin is a receptor "decoy" that inhibits neuraminidase. Several known antiviral agents, including ascorbic acid, inhibit neuraminidase. It is proposed that ascorbic acid may mediate an antiviral effect through the incorporation of ascorbic acid or some derivative of ascorbic acid as a part of the sialoresponsin molecule. Whether ascorbic acid works alone as a pharmacological inhibitor, or is incorporated in sialoresponsin as a physiological inhibitor; it may be useful against pathogens that employ neuraminidase.

A gene apparently determining the extent of sialylation of lysosomal alpha-mannosidase in mouse liver

Organ-specific electrophoretic heterogeneity of lysosomal alpha-mannosidase has been observed within individual strains of inbred mice. Polymorphism between C57BL/6J and CBA/J for liver lysosomal alpha-mannosidase is determined by a single genetic locus on chromosome 5 and appears to be the result of differences in sialylation of the lysosomal enzyme. Two different patterns of expression of development of the liver electrophoretic forms have been observed.

Isoelectric-focusing behavior of acid hydrolases in rat kidney lysosomes. Effects of the pH gradient, autolysis and neuraminidase

Isoelectric focusing was used to study the multiple forms of acid phosphatase, arylsulfatase, beta-glucuronidase and beta-N-acetylhexosaminidase in lysosomes isolated from rat kidney. The isoelectric points of the main protein and hydrolase peaks were 1-1.5 units lower when electrofocusing was done in a pH 3-10 gradient than in a pH 10-3 gradient, apparently because the lysosomal constituents aggregated strongly at their isoelectric points and tended to settle somewhat in the gradient due to gravity. In the extended pH gradient the acidic form of each hydrolase occurred as asingle, relatively discrete peak. However, when pooled acidic fractions were refocused in a restricted pH gradient (pH 6-3 or 3-5) multiple acidic enzyme and protein components were resolved with isoelectric points between 2.7 and 5.1. When autolysis was minimized by extracting lysosomal fractions at alkaline pH (0.2% Triton X-100, 0.1%p-nitrophenyloxamic acid, 0.1 M glycine buffer, pH9) and including 0.1%p-NITROPHENYLOXAMIC ACID, AN INHIBITOR OF LYSOSOMAL NEURAMINIDASE AND CATHEPSIN D, in the pH gradient, arylsulfatase, beta-glucuronidase and beta-N-acetylhexosaminidase occurred in two forms, an acidic form with an isoelectric point of about 4.4, and a basic form with an isoelectric point close to 6.2, 6.7 and 8.0, respectively. Acid phosphatase occurred in three forms with isoelectric points of 4.1, 5.6 and 7.4. When some autolytic digestion was permitted by extracting lysosomal fractions in an acidic medium (0.2% Triton X-100, 0.1 M sodium acetate buffer, pH 5.2) AT 0-4DEGREES C and omitting p-nitrophenyloxamic acid from the gradient, the acidic form of beta-glucuronidase and the intermediate form of acid phosphatase were lost, the isoelectric points of the acidic forms of acid phosphatase, arylsulfatase and beta-N-acetylhexosaminidase were increased 0.6-1.2 units, and the isoelectric point of the basic forms of acid phosphatase, arylsulfatase and beta-glucuronidase was increased 0.5 unit. When lysosomal extracts were incubated with bacterial neuraminidase before electrofocusing, the acidic forms of acid phosphatase, arylsulfatase and beta-glucuronidase were largely lost, the isoelectric point of the acidic form of beta-N-acetylhexosaminidase was increased from 4.5 to 6.4, and the isoelectric points of the basic forms of all four hydrolases were increased 0.5-1.5 units. Autoincubation of lysosomal extracts in vitro at pH 5.2 PRODUCED SIMILAR, THOUGH LESS MARKED, effects. cont'd

Autolysis of glycoproteins in rat kidney lysosomes in vitro. Effects on the isoelectric focusing behaviour of glycoproteins, arylsulphatase and beta-glucuronidase

1. Rat kidney lysosomal glycoproteins, prelabelled in the N-acetylneuraminic acid and polypeptide portions with N-acetyl[(3)H]mannosamine and [(14)C]lysine, or with N-acetyl-[(14)C]glucosamine, were incubated under various conditions. Autolytic cleavage of labelled N-acetylneuraminic acid and peptide was maximum at pH5.0. 2. N-Acetylneuraminic acid was released more rapidly than peptide during incubation at 37 degrees or 4 degrees C at pH5. p-Nitrophenyloxamic acid, an inhibitor of bacterial neuraminidase (Edmond et al., 1966), inhibited the cleavage of N-acetylneuraminic acid and peptide, and also inhibited cathepsin D activity. 3. Galactono-, mannono-, and glucono-lactone, inhibitors of the corresponding glycosidases, blocked the autolytic cleavage of N-acetyl[(14)C]glucosamine and protein without inhibiting beta-N-acetylhexosaminidase or cathepsin D activity. These findings suggest that the carbohydrate side chains protect the polypeptide portion of the lysosomal glycoproteins against proteolytic attack by lysosomal cathepsins. 4. In electrofocusing experiments, autolysis was minimized by adding 0.1% p-nitrophenyloxamic acid to the media used for extraction and electrofocusing, and by maintaining an alkaline pH (pH8.8-9) during extraction and dialysis. Arylsulphatase occurred in two forms with pI values of 4.4 and 6.4-6.7, and beta-glucuronidase in two forms with pI values of 4.4 and 6.1. When [(14)C]lysine and N-acetyl[(3)H]mannosamine were given to rats 1.5 and 1 h before killing, (14)C and (3)H were largely restricted to highly acidic glycoprotein species with pI values of 2.1-5.1. 5. When a lysosomal extract was adjusted to pH5 and incubated at 20 degrees C for 16h and then at 37 degrees C for 1 h before electrofocusing, 32 and 58% of the labelled peptide and N-acetylneuraminic acid was cleaved and the pI values of the labelled glycoproteins were markedly increased. About 80% of the acidic form of arylsulphatase and beta-glucuronidase was recovered with the basic form, and the pI of the basic form of both enzymes rose to 7.0. Similar, though less marked changes, were observed when a lysosomal extract was kept at pH5 for 2h at 4 degrees C before electrofocusing. 6. When an acidic lysosomal fraction (pI4.2-4.6) was incubated at pH5 for 2.5h and refocused, 80% of the arylsulphatase now occurred in two forms with pI values of 5 and 6.4. When a basic lysosomal fraction (pI5.8-6.4) was similarly incubated, the pI of arylsulphatase increased from 6.4 to 7.2. The relative increase in pI of arylsulphatases was accompanied by a proportional loss of N-acetylneuraminic acid from the glycoprotein associated with these forms. 7. These experiments show that lysosomal glycoproteins and two representative hydrolases, when exposed to a mildly acidic pH, readily undergo autolytic degradation and their pI values increase. These observations may have a bearing on the origin of the molecular heterogeneity of the lysosomal enzymes.

Studies on human thyroxine-binding globulin. VI. The nature of slow thyroxine-binding globulin

A model system utilizing a highly purified partially desialylated thyroxine-(T(4)) binding protein (STBG) was studied. STBG was prepared by the same affinity chromatographic method we have reported for preparation of highly purified T(4)-binding globulin (TBG). The necessary prerequisite for preparation of STBG was the use of T(4)-substituted Sepharose which had been repeatedly exposed to large volumes of serum for purification of TBG. STBG moved more slowly on cellulose acetate electrophoresis than TBG but had the same molecular weight and antigenic determinants as TBG. It bound T(4) with a 1: 1 molar ratio but its affinity for T(4) was about 10 times less than that of TB. STBG had about onefourth the sialic acid content of TBG and the electrophoretic mobility of this protein was similar to that of a T(4)-binding protein with a mobility slower than that of TBG which has been seen in the electrophoretic patterns of some normal human serums and in serums of patients with hepatic cirrhosis and which does not appear to be an artifact caused by storage and freezing of serum. This fourth slowly migrating T(4)-binding region in electrophoretograms of cirrhotic serums is completely abolished by prior incubation with rabbit antiserum to TBG. The in vitro production of partially desialylated TBG from T(4)-Sepharose which had been previously exposed to large volumes of serum may be due to adsorption of neuraminidases to the Sepharose either directly from serum or as the result of bacterial contamination. Partial desialylation of TBG in vivo may be an early step in the catabolism of this protein.