in the invention , the moiety which includes the functional group may be linked directly to the polysialic acid unit or , more conveniently , may be linked via a difunctional organic group , such as an alkane diyl group , an arylene group or an oligo ( alkyleneoxy ) alkane group , or alternatively an oligo peptidyl group . the linkage to the polysialic acid ( from the linker or the moiety including the functional group may be a secondary amine , a hydrazone , an alkyl hydrazine an ester or a peptidyl group . the moiety may be generated by reaction of a polysialic acid substrate with a heterobifunctional reagent . the process forms a further aspect of the invention . reagents useful for introducing the selected functional groups are commercially available . a compound which will introduce a maleimide group onto an amine moiety without introducing any additional linker moiety is n - methoxy - carbonyl - maleimide . generally the reagents include a second functional group for reaction with a group on the polysialic acid which may be the aldehyde group described above , or an amine group . suitable second functional groups include n - hydroxy succinimide esters and their sulfosuccimide analogues and hydrazides . preferably the compound is a n - maleimido - alkanoic acid hydrazide or an n - maleimidoarylalkanoic acid hydrazide , i . e ., a compound having the general formula r is alkane - diyl , arylene or aralkylene alkarylene , alkylene - oxaalkylene , or alkylene - oligooxa - alkylene or alkyl - oligopeptidyl - alkyl group ; and preferably r is c 1 - 6 alkanediyl , c 2 - 3 - alkyl - oxa - c 2 - 3 - alkylene , c 2 - 3 alkyl - oligo ( oxa - c 2 - 3 alkylene ), or c 2 - 6 alkylene phenyl . preferably x is n - maleimido or orthopyridyldisulphide . preferably y is a hydrazide or a hydroxyl succinimide . compounds which may be reacted with an aldehyde group , and which include a linker moiety and introduce a maleimide group include n -[( β - maleimidopropionic acid ] hydrazide and 4 -( 4 - n - maleimidophenyl ) butyric acid hydrazide . the hydrazide group reacts with the aldehyde to form a stable hydrazone group . a suitable heterobifunctional compound which includes an oligo ( ethyleneoxy ) ethylene group is a compound comprising a polyethylene glycol ( poly ( ethyelenoxy )) group with , at one end , n - hydroxy succinimide ( nhs ) group and at the other end the functional group . the nhs group reacts with amine groups to form stable amide linkages . heterobifunctional polyethyleneglycols with nhs at one end and either vinylsulphone or maleimide at the other end are available . other examples of heterobifunctional reagents include , 3 -( 2 - pyridyldithio ) propionyl hydrazide , n - succinimidyl - 3 -[ 2 - pyridyldithio ] propionate , succinimidyl - h -[ n - maleimidomethyl ] cyclohexane - 1 - carboxylate ), m - maleimidobenzoyl - n - hydroxysuccinimide ester , n - succinimidyl -[ 4 - iodoacetyl ] amino benzoate , n -[ gamma - maleimidobutyryloxy ] succinimide ester , n -[ epsilon - maleimidocaproyloxy ] succinimide ester and n - succinimidyl iodoacetate . other reagents are available from pierce biotechnology and shearwater corporation ( polyethylene glycol - based ). sialic acids ( also known as nonulosonic acids ) are members of a family of amino containing sugars containing 9 or more carbon atoms . the most important of the sialic acids is n - acetylneuraminic acid ( also known as 5 -( acetylamino )- 3 , 5 - dideoxy - d - glycero - d - galacto - nonulosonic , lactaminic acid and o - sialic acid ) which has the formula : polysialic acids may be linked 2 → 8 and / or 2 → 9 , usually in the α - configuration . in some polysialic acids the linkages are alternating 2 → 8 and 2 → 9 . the invention is also of utility for heteropolymeric polysaccharides comprising glycosyl units other than sialic acid units . polysialic acids are generally found to be non - toxic and substantially non - immunogenic . furthermore the biodegration units , sialic acid , is not known to be toxic and , indeed sialic acids are widely found in animal proteins and cells , including blood cells and circulating proteins . polysaccharide compounds containing many sialic acid units are polysaccharides produced by escherichia coli , moraxella nonliquifaciens , pasteurella aeroginosis and neisseria meningitidis or derivatives thereof . for instance colominic acid derived ( by hydrolysis to shorten the chain lengths ) from e . coli ki comprises α 2 → 8 linked sialic acid units . polysaccharide from e . coli k92 strain comprises alternating 2 → 8 and 2 → 9 linked sialic acid units . polysaccharide c of n . meningitidis group c has 2 → 9 linked sialic acid units . one group of polysaccharide compounds which has been found to be of particular utility in the invention is group b polysaccharides . these compounds are produced by n . meningitidis , m . nonliquifaciens , p . aeroginosis a2 and e . coli k1 . these compounds comprise a polysaccharide component comprising sialic acid residues and a phospholipid component . the sialic acid residues are linked ( 2 → 8 )- α , the naturally occurring polymer consisting of about 200 residues . some of the glycolipid molecules , especially the high molecular weight compounds appear to have a covalently attached phospholipid at the reducing end of the polysaccharide component . it is preferable for the bacteria from which the polysaccharide compound is derived to be non - pathogenic for convenience during production . it is particularly suitable therefore for the polysaccharide to be derived from a non - pathogenic strain of e . coli such as e . coli k92 or , preferably , k1 which is non - immunogenic . e . coli k92 and k1 isolates are well - known and any such type of any such strains can be used as sources of suitable polysaccharide . preferably the polysialic acid should have at least 2 , preferably at least 5 , more preferably at least 10 , for instance more than 50 sialic acid units . according to the invention , there is also provided a conjugate of a protein and the novel activated polysaccharide . the novel compound comprises a protein with at least one cysteine unit and , linked through a thioether bond to the side chain of a cysteine unit , a polysialic acid through a moiety joined at one or both terminal unit of the polysialic acid . where the polysialic acid derivative was a n - maleimido group the moiety will include a n - succinimidyl group , with the thioether linked at the α - carbon atom . preferably the moiety also comprises a secondary amine , a hydrazone or an amide bond . there is also provided in the invention a new process in which a polysialic acid is reacted with a heterobifunctional reagent having a first functional group for reaction with sulfhydryl groups and a second functional group different to the first group whereby the said second functional group reacts with a terminal unit of the polysialic acid to form a covalent bond therewith and form a capable of reaction with a sulfhydryl group functional polysialic acid . in one embodiment the second functional group is a nucleophilic group , preferably hydrazine . this is of particular utility where the polysialic acid comprises an aldehyde group in the terminal unit whose carbonyl group is attacked by the nucleophilic group . in another embodiment of the process the second functional group is electrophilic such as an n - alkoxyl carbonyl - imide such as n - hydroxysuccinimide ester or sulphosuccinimide ester , or carbodiimide . the terminal group in such cases is preferably nucleophilic such as amine . in the process it is preferred that the reagent comprises a bifunctional organic group linking the first and second functional groups . preferably the bifunctional organic group is selected from a c 2 - 18 - alkanediyl group , an arylene group , an oligo peptide and an oligo ( alkoxy ) alkyl group . most usefully the process involves a subsequent step in which the functional polysialic acid is reacted with a polypeptide or a protein having at least one free and unprotected cys unit whereby the functional group forms a thioether linkage with the thiol group of a cys unit to form a polysialylated polypeptide or protein . the process is particularly suitable where the protein contains a single cys unit , whereby site - controlled derivatisation is achieved . colominic acid aldehyde ( cao ) produced according to wo - a - 9222331 ( 100 mg , 4 . 4 × 10 − 6 mol ) was dissolved into 500 μl 0 . 1 m sodium acetate , to this 5 molar equivalents of n -[ β - maleimidopropionic acid ] hydrazide ( 6 . 5 mg , 2 . 2 × 10 − 5 mol ) was added . this mixture was then vortex mixed and wrapped in foil and allowed to incubate at 37 ° c . for 2 h on a rotary mixer . the polymer was then precipitated by the addition of 2 volumes ( 1 . 0 ml ) of ethanol . the precipitate was collected by centrifugation ( 13 , 000 rpm , 2 min ) in a bench top microcentrifuge . the supernatant was discarded and the pellet dissolved in 500 μml 0 . 1 m acetate . this process was repeated a further 2 times and the final pellet dissolved in deionised water and freeze dried overnight . in this assay cysteine is reacted with the maleimide on the polymer preventing further reaction with ellman &# 39 ; s reagent ( 5 , 5 ′- dithiobis ( 2 - nitrobenzoic acid )) which contains a disulphide which forms an intense yellow colour when it is substituted for a thiol not adjacent to an aromatic ring . thus maleimide content can be calculated by measuring the inhibition of reaction between cysteine and ellman &# 39 ; s reagent assay . first an initial stock of cysteine at 12 × 10 − 3 m ( 0 . 145 mg / ml ) was prepared in pbs . in a clean microtitre plate 100 μl volume doubling dilutions from 12 × 10 − 3 m to 0 . 375 × 10 − 3 m were made from row b to row h . in row a , 100 μl pbs was used as a zero standard . samples of ca and ca - maleimide ( cam ) were prepared at 5 or 10 mg / ml and 100 μl of each sample added to duplicate columns of the cysteine dilutions . in one set , 100 μl pbs without any ca was added as a control . the plate was covered and allowed to incubate at 37 ° c . for 1 hr . after this time 20 μl of ellman &# 39 ; s reagent ( 4 mg / ml in pbs ) was added to each well and the plate incubated in the dark at room temperature for 15 min . absorbance was the measured in wells at 405 nm . standard curves were then plotted for the samples and the amount of maleimide present calculated from inhibition of the signal . 1 . 4 thiolation of fab and conjugation to colominic acid maleimide ( cam ) in the first step a thiol group is introduced into a model protein by thiolation of an amine of lysine . ovine fab ( anti desipramine / norityrptaline , 4 mg , 7 . 2 × 10 − 6 mol ) was dissolved in 0 . 25 ml pbs + 10 mm edta ; to this was added 0 . 498 mg 2 - iminothiolane ( 2 - it , traut &# 39 ; s reagent 50 mol equiv 3 . 6 × 10 − 6 mol ) in 0 . 25 ml of the same buffer . the tube was wrapped in foil and left to incubate stirring end over end for 1 hr . at 37 ° c . thiolated fab was purified from free 2 - it traut &# 39 ; s reagent by gel filtration ( pd - 10 ) and 0 . 5 ml fractions assayed for presence of protein ( bca assay ) or thiol ( ellman &# 39 ; s assay ). the first eluting peak containing both was pooled and protein and thiols quantified . to thiolated fab ( 3 . 6 mg , 6 . 6 × 10 − 8 mol ) in 2 ml pbs / edta 22 . 5 mg cam was added ( 9 × 10 − 7 mol , 15 molar equiv ). the tube was sealed and allowed to incubate at 37 ° c . for 1 h whilst gently mixing . the resulting conjugate was then purified according to accepted protocols to remove free cam . both ca and protein content were assayed on the conjugate to calculate conjugation ratio . control reactions were carried out with ca as a negative control . several batches of cam - fab were prepared with various degrees of thiolation but maintaining the 15 : 1 ratio of cam : fab . results are shown in table 1 below : the results show that in all control wells ( samples of thiolated fab ) the migration of the sample is similar to that for fresh fab ( below that of the 50 kda marker ) indicating no cross linking of fab molecules during the process of conjugation . in the replicate lanes there is considerable band broadening with maximum intensity between the 98 and 250 kda markers which typically indicates an increase in mass which is indicative of a of polysialylated product . to e . coli β - galactosidase ( β - gal ) 5 . 0 mg , 4 . 3 × 10 − 8 mol ) in 1 ml pbs 15 mg cam was added ( 6 . 59 × 10 − 7 mol , 15 molar equiv ). the tube was sealed wrapped in foil and allowed to incubate at room temperature for 1 hr . whilst gently mixing . the resulting conjugate was analysed by sds page and then purified according to accepted protocols to remove free cam . samples were assayed for polymer and protein content as outlined elsewhere . control reactions were carried out with ca as a negative control . standards from 60 μg / ml to 3 . 75 μg / ml of fresh β - galactosidase were prepared in pbs . sample of cam - β - gal were diluted to 60 μg / ml in the same buffer . enzyme activity of the conjugates was measured as follows : in a microtitre plate , to 100 μl of sample or standard was added 100 μl of all - in - one ™ β - gal substrate ( pierce ). the plate was incubated at 37 ° c . for 30 min and absorbance read at 405 nm . a calibration curve was prepared from the standards and the activity of the samples calculated from the equation for the linear regression of the curve . from the protein and polymer assays the conjugation ratio was determined to be 1 . 23 cam : 1 β - gal . there was also a corresponding increase in apparent molecular mass from the sds page of the samples ( fig2 ). enzyme activity in the purified sample was calculated to be 100 . 4 % compared to the free enzyme . oxidised colominic acid at ( cao ) 10 - 100 mg / ml was dissolved in 2 ml of deionised water with a 300 - fold molar excess of nh 4 cl , in a 50 ml tube and then nacnbh 3 ( 5 m stock in 1 n naoh ( aq ) was added at a final concentration of 5 mg / ml . the mixture was incubated at room temperature for 5 days . a control reaction was also set up with colominic acid ( a ) instead of cao . product colominic acid amine derivative was precipitated by the addition of 5 ml ice - cold ethanol . the precipitate was recovered by centrifugation at 4 , 000 rpm , 30 minutes , room temperature in a benchtop centrifuge . the pellet was retained and resuspended in 2 ml of deionised water , then precipitated again with 5 ml of ice - cold ethanol in a 10 ml ultracentrifuge tube . the precipitate was collected by centrifugation at 30 , 000 rpm for 30 minutes at room temperature . the pellet was again resuspended in 2 ml of deionised water and freeze - dried . the tnbs ( picrylsulphonic acid or 2 , 4 , 6 - tri - nitro - benzene sulphonic acid ) assay was used to determine the amount of amino groups present in the product . in the well of a microtitre plate tnbs ( 0 . 5 μl of 15 mm tnbs ) was added to 90 μl of 0 . 1 m borate buffer ph 9 . 5 . to this was added 10 μl of a 50 mg / ml solution of ca - amide the plate was allowed to stand for 20 minutes at room temperature , before reading the absorbance at 405 nm . glycine was used as a standard , at a concentration range of 0 . 1 to 1 mm . tnbs trinitrophenylates primary amine groups . the tnp adduct of the amine is detected . testing the product purified with a double cold - ethanol precipitation using the tnbs assay showed close to 90 % conversion . ca - amine ( 17 mg ) was dissolved in 1 ml deionised water , to this was added 6 mg methoxy - carbonyl - maleimide ( mcm ). the mixture was left to react at room temperature for 30 min . to the sample 1 100 μl water and 200 μl acetonitrile was added and then incubated at room temperature for 4 h , after which 300 μl chcl 3 was added the tube shaken and the aqueous fraction collected . then the fraction was purified over a pd - 10 column to remove small molecules . the eluate was freeze dried and assayed for maleimide content . the molar concentration of maleimido was 44 mol %. to 40 mg colominic acid amine ( 85 mol % amine ) as ( described in example 2 . 1 ) dissolved in 1 ml of pbs ph 7 . 4 was added 5 mg of n - succinimidyl iodoacetate ( sia ). the mixture was left to react for 1 h at 37 ° c ., after which excess sia was removed by gel filtration over a 5 ml hightrap ™ desalting column ( ap bioscience ) eluted with pbs . 0 . 5 ml fractions were collected from the column and samples from each fraction tested for colominic acid content ( resorcinol assay ) and reactivity with cysteine indicating iodide ( ellman &# 39 ; s assay ). fractions positive for both iodide and ca were pooled . to e . coli β - galactosidase ( 5 . 0 mg , 4 . 3 × 10 − 8 mol ) in 1 ml pbs 15 mg cai was added ( 6 . 59 × 10 − 7 mol , 15 molar equiv ). the tube was sealed wrapped in foil and allowed to incubate at room temperature for 1 hr . whilst gently mixing . the resulting conjugate was analysed by sds page and then purified according to accepted protocols to remove free cai . samples were assayed for polymer and protein content . as outlined elsewhere . control reactions were carried out with ca as a negative control . all samples were analysed for β - gal activity as example 1 . 8 above . fractions 3 - 6 were positive for both polymer and iodoacetate and were pooled . the sds page ( 4 - 12 % bis / tris gel ; fig2 ) showed an increase in apparent molecular mass for samples incubated with the iodoacetamide derivative but not with control polymer . from the protein and polymer assays the conjugation ratio was determined to be 1 . 63 cai : 1 β - gal . β - gal activity was calculated to be 100 . 9 % for the conjugated sample , compared to the free enzyme . 4 . 1 synthesis : non - reducing end activation with 4 ( 4 - n - maleimidophenyl ) butyric acid hydrazide ( ca - mbph ) and 3 -( 2 - pyridyldithio ) propionyl hydrazide ( ca - pdph ) colominic acid aldehyde ( cao ; 22 . 7 kda , camida , ireland ) produced according to wo - a - 9222331 ( 73 mg for mbph , tube a ; 99 . 3 mg for pdph ; tube b ) was dissolved individually into 800 ml 0 . 1 m sodium acetate ( ph 5 . 5 ). to tube a 15 mg of mbph ( 15 : 1 linker : ca ratio ) dissolved in 200 μl of dmso was added . to tube b 15 mg ( 15 : 1 linker : ca ratio ) of pdph dissolved in 200 of dmso was added . the ph was adjusted to 5 . 5 in each case for each reaction vessel . these mixtures were then vortex mixed and wrapped in foil and allowed to incubate at 37 ° c . for 2 hr . on a orbital mixer . each polymer solution was purified by gel filtration ( pd 10 column ) eluting with pbs ph 7 . 4 and the 1 ml fraction containing ca ( by resorcinol assay ) collected . these sample were freeze dried overnight and assayed for maleimide content as described in example 1 . 2 . 4 . 2 synthesis : reducing end activation with 4 ( 4 - n - maleimidophenyl ) butyric acid hydrazide ( mbph - ca ), ( 2 - pyridyldithio ) propionyl hydrazide ( pdph - ca ) and n - β - maleimidopropionic acid hydrazide ) hydrazide ( bmph - ca ) the preparations were carried out as follows all at molar ratios of 25 : 1 linker to ca : colominic acid ( ca ; 22 . 7k ; 73 mg for mbph , tube a ; 99 . 3 mg for pdph ; tube b and 76 . 6 mg for bmph ; tube c ) was dissolved individually into 800 ml 0 . 1 m sodium acetate ( ph 5 . 5 ) separately . to tube a , 25 mg of mbph ( dissolved in 200 μl of dmso ), to tube b 25 mg of pdph ( dissolved in 200 μl of dmso ) and to vial c 25 mg of bmph ( dissolved in 200 μl of sodium acetate buffer ) were added . the ph of the reaction mixture was adjusted to 5 . 5 . these mixture was then vortex mixed , wrapped in foil and allowed to incubate at 37 ° c . for 72 h on an orbital mixer . each polymer solution was purified by gel filtration ( pd 10 column ) eluting with pbs ph 7 . 4 and the 1 ml fraction containing ca ( by resorcinol assay ) collected . these sample were freeze dried overnight and assayed for maleimide content as described in example 1 . 2 . the molar concentration of maleimido was 49 . 0 and 35 . 0 mol % for mbph and pdph respectively on the non - reducing ( highly reactive ) end . the molar concentration of maleimido was 41 . 5 , 32 . 5 and 48 . 3 mol % for mbph , pdph and bmph respectively on the reducing end ( weakly reactive ). the values on the reducing end are average of two values in each case . 4 . 4 conjugation of β - galactosidase ( β - gal ) to maleimide activated colominic acids ( reducing end and non - reducing end ) to β - gal ( 1 . 0 mg ; 1 ml in pbs / edta ) in separate tubes , a 15 molar excess of each maleimide activated ca ( mbph , pdph and bmph on non - reducing or reducing end , from above examples ) was added separately . each tube was sealed and allowed to incubate at 37 ° c . for 1 h whilst gently mixing . the resulting conjugate was then purified according to accepted protocols to remove free activated polymer . all samples were analysed by sds page and for β - gal activity as in example 1 . 8 . the results ( fig3 ) show that in all control wells ( with β - gal ) the migration of the sample is similar to that for fresh β - gal . in the conjugate lanes there is considerable band broadening with maximum intensity between the 98 and 250 kda markers which typically indicates an increase in mass which is indicative of a polysialylated - β - gal . β - gal activity was calculated to be 91 . 0 - 106 % for the conjugated samples .
a polysialic acid compound is reacted with a hetero - bifunctional reagent to introduce a pendant functional group for site - specific conjugation to sulfhydryl groups , for instance side chains of cysteine units in drugs , drug delivery systems , proteins or peptides . the functional group is , for instance , an n - maleimide group .