( aa ) n is a blocked or unblocked , thionated or unthionated amino acid whenever n = 1 , and ( aa ) n is a peptide consisting of blocked or unblocked , thionated or unthionated amino acids whenever n is greater than one . as used herein the term &# 34 ; thionated or unthionated amino acid &# 34 ; is defined by the following formula : ## str1 ## wherein r is defined by the particular amino acid employed , and z is oxygen or sulfur . when the term &# 34 ; thionated &# 34 ; is used herein in reference to an amino acid z is sulfur , and when the term &# 34 ; unthionated &# 34 ; is used z is oxygen . n is in the range of 0 to 12 , and more preferably in the range of 0 to 4 . aaψ [ cs ] is a blocked or unblocked amino acid whose alpha carbonyl oxygen has been replaced with a sulfur atom ( that is , the alpha carbonyl is a thiocarbonyl ), and whose alpha amine forms a peptide bond with the alpha carbonyl of the adjacent amino acid of the ( aa ) n moiety whenever n is greater than or equal to one . aaψ [ cs ] is defined by the formula : ## str2 ## wherein r is defined by the particular amino acid employed . w is a chromogenic or fluorogenic leaving group , such as 5 - aminoisophthalic acid dimethylester ( also referred to herein as aie ), 7 - amino - 4 - trifluoromethylcoumarin ( also referred to herein as afc ), 4 - methoxy - 2 - naphthylamine ( also referred to herein as mna ), 7 - amino - 4 - methylcoumarin ( also referred to herein as amc ), p - nitroaniline ( also referred to herein as pna ), cresyl violet , rhodamine , or the like . preferably w is a fluorogenic leaving group , such as aie , afc , mna , amc , pna , or the like . most preferably w is aie . the invention includes salts of the compounds defined by formula i . in particular , such salts include hydrochloride , dihydrochloride , hydrobromide , dihydrobromide , trifluoroacetic , ditrifluoroacetic , and like salts . the blocking groups which may be present on aaψ [ cs ] or on the amino acid or peptide represented by ( aa ) n are those well known in the art of peptide synthesis . for example , a listing of suitable peptide blocking groups is found in gross et al ., eds ., the peptides , vol . 3 ( academic press , new york , 1981 ). the particular choice of blocking group used in the compounds of the invention depends on several factors , including the blocking group &# 39 ; s affect on enzyme specificity , its affect on substrate solubility , and its utility during synthesis . suitable blocking groups include but are not limited to carbobenzoxy ( cbz ), benzoyl , t - butoxycarbonyl ( boc ), glutaryl , p - tolylsulfonyl ( tos ), methoxysuccinyl ( meosuc ), succinyl , glutaryl , and certain d - isomers of naturally occurring l - amino acids , including but not limited to d - proline , d - valine , and d - alanine . cysteine proteases detectable by compounds of the present invention include , but are not limited to , cathepsins b , h , l , n , s , t , i , j , and k , cancer procoagulant , papain , chymopapain , pyroglutamyl peptide hydrolase , calpain i and ii , gamma - endorphin generating endopeptidase , viral cysteine protease , and the like . selection of the most suitable substrate from the set defined by formula i for detecting a given cysteine protease depends on the protease &# 39 ; s specificity requirements . generally , different cysteine proteases have different activities with respect to a given peptide bond , and the activities depend critically on the sequence of amino acids making up the peptide adjacent to the scissle bond . consequently , picking the best compound of formula i for detecting a particular cysteine protease may require some routine experimentation to determine the optimal peptide moiety for the protease . preferably whenever the cysteine protease is cathepsin b the substrate is cbz - val - lys - lys - argψ [ cs ]- w , cbz - arg - argψ [ cs ]- w , suc - tyr - metψ [ cs ]- w , beta - ala - tyr - metψ [ cs ]- w , d - leu - tyr - metψ [ cs ]- w , or cbz - ala - arg - argψ [ cs ]- w ; whenever the cysteine protease is cathepsin l the substrate is cbz - phe - argψ [ cs ]- w , cbz - arg - argψ [ cs ]- w , suc - tyr - metψ [ cs ]- w , cbz - ala - arg - arg ψ [ cs ]- w , beta - ala - tyr - metψ [ cs ]- w , or d - leu - tyr - met ψ [ cs ]- w ; whenever the cysteine protease is cathepsin h the substrate is argψ [ cs ]- w , cbz - arg - argψ [ cs ]- w , suc - tyr - metψ [ cs ]- w , cbz - ala - arg - argψ [ cs ]- w , beta - ala - tyr - metψ [ cs ]- w , or d - leu - tyr - metψ [ cs ]- w ; whenever the cysteine protease is pyroglutamyl peptide hydrolase the substrate is pyrogluψ [ cs ]- w ; whenever the cysteine protease is cathepsin e the substrate is cbz - gly - gly - argψ [ cs ]- w ; whenever the cysteine protease is cathepsin m the substrate is argψ [ cs ]- w ; and whenever the cysteine protease is papain the substrate is cbz - lysψ [ cs ]- w , meosuc - phe - thr ( obzl ) ψ [ cs ]- w , or cbz - phe - argψ [ cs ]- w . here the conventional three letter designations for the amino acids have been used , e . g . &# 34 ; iupac - iub commission on biochemical nomenclature symbols for amino - acid derivatives and peptides recommendations ( 1971 ),&# 34 ; j . biol . chem ., vol . 247 , pgs . 977 - 983 ( 1972 ). generally compounds of the invention are synthesized by first forming an amino acid - leaving group conjugate , which may include suitable blocking groups . next , the conjugate is thionated , preferably using lawesson &# 39 ; s , or like reagent , to form a thioneamide , e . g . see cherkasov et al ., &# 34 ; organothiophosphorus reagents in organic synthesis ,&# 34 ; tetrahedron , vol . 41 , pgs . 2567 - 2624 ( 1985 ); clausen et al ., tetrahedron , vol . 37 , p . 3637 ( 1981 ); and yoe et al ., tetrahedron , vol . 40 , pgs . 2047 - 2052 ( 1984 ). additional compounds of the invention are synthesized by linking a thionated or unthionated amino acid to the alpha amine of the thioneamide conjugate , and then to the alpha amine of the thionated or unthionated amino acid just attached , and so on , until a peptide thioneamide of desired length is obtained . the type of thionated amino acid linked to the leaving group , and the number and ordering of the thionated or unthionated amino acids added thereto in large part determine the enzyme specificity for the substrate . as indicated above , any combination of amino acids can be employed to obtain the desired specificity . in some cases it may be desirable to employ one or more thionated amino acids in the ( aa ) n moiety to prevent degradation of the substrate by serine proteases which may be present in a sample fluid . such thionated peptides can be synthesized following the procedures disclosed in thorsen et al ., tetrahedron , vol . 39 , pgs . 3429 - 3435 ( 1983 ); or in clausen et al ., j . chem . soc . perkin trans . i , pgs . 785 - 798 ( 1984 ). accordingly , both of these articles are incorporated by reference . the method of the present invention for determining the presence of an enzyme in an enzyme - containing analyte comprises contacting the analyte with a substrate which can be hydrolyzed with an enzyme . such analyte is usually a natural biological fluid , such as blood , serum , urine , tissue homogenate , or the like , but it can also be a synthetic solution used for quality control or as a reference standard . in any case the amount of substrate contacted with the sample fluid must be great enough so that a detectable fluorescent or colorimetric signal is generated for the reaction conditions employed . this amount of substrate is referred to herein as an effective amount . the analyte - substrate mixture is incubated under enzyme hydrolyzing conditions . the ph of the mixture is generally in the range of the normal physiological environment of the enzyme , and thus can vary from one enzyme to another . the ph of the mixture is conveniently controlled by mixing the analyte and substrate in an appropriate buffering agent , such as n - tris ( hydroxymethyl ) methyl - 2 - aminoethane sulfonic acid ( tes ), or the like . in some cases , the concentration of certain divalent metal ions , such as ca ++ , mg ++ , or the like , must be regulated for proper enzyme action . the term &# 34 ; buffering agent &# 34 ; comprehends the inclusion of agents , e . g . edta , to effect such regulation when needed . the temperature at which the enzyme hydrolysis is effected is not critical , and may fall within a broad range , provided that the temperature is high enough to ensure enzyme activity , but not too high to cause degradation or other harmful reactions involving the substrate , enzyme , or other components of the mixture . fluorimetric or spectrophotometric determination of the liberated fluorophor or chromophor can be either a rate determination or and end point determination . rate determinations are preferred because they are generally more sensitive and precise . in a rate determination , the fluorescence and / or absorption of the substrate - analyte mixture may be determined promptly after the analyte is contacted with the substrate . in an end point determination , enzyme hydrolysis is allowed to proceed for a predetermined length of time , e . g . for about 5 to 60 minutes , preferably for about 15 - 30 minutes . such reaction time is selected so that a sufficient quantity of fluorophor and / or chromophor is released as to provide an acceptable degree of accuracy for the assay . standard fluorimeters or spectrophotometers are used to make the fluorescence intensity measurements and / or absorption measurements , e . g . see pearson et al ., clin . chem ., vol . 27 , pgs . 256 - 262 ( 1981 ); udenfriend , fluorescence assay in biology and medicine , vol . i and ii ( academic press , new york , 1961 and 1967 ); or u . s . pat . no . 4 , 388 , 233 , to name just a few references describing appropriate fluorimetric or colorimetric detection apparatus . substrates of the invention are useful for a variety of analytical techniques . they can be utilized in biological studies to determine the presence of cysteine proteases in single cells or tissues , and they are useful to determine the presence of cysteine proteases in biological fluids of clinical importance . the substrates can be used as indicators in connection with various chromatographic or electrophoretic techniques . the appropriate substrate can be applied to the chromatographic or electrophoretic medium to indicate the location and / or density of a previously separated enzyme . finally , the substrates of the invention can be used to classify enzymes as cysteine proteases or not . the following examples serve to illustrate the present invention . the concentrations of reagents , choice of temperatures , choice of solvents , and values of other variable parameters are only to exemplify the practice of the present invention and are not to be considered as limitations thereof . mtr - cl and cbz - arg ( mtr )- oh were synthesized by the method of fujino et al ., chem . pharm . bull ., vol . 29 , pgs . 2825 - 2831 ( 1981 ). here &# 34 ; mtr &# 34 ; refers to 4 - methoxy - 2 , 3 , 6 - trimethylbenzenesulfonyl , which is employed as a blocking group for the guanidino moiety of arginine . mixed anhydride coupling method was employed to obtain cbz - arg ( mtr )- aie ( r f 1 1 0 . 57 ). cbz - arg ( mtr )- oh ( 5 . 00 g , 9 . 62 mmol ) was dissolved in 50 ml of tetrahydrofuran ( thf ) containing 1 eq n - methylmorpholine ( nmm ), and cooled to - 15 ° c . isobutyl chloroformate ( ibcf ) was then added and the mixture stirred for 2 . 5 minutes , after which aie ( 2 . 01 g , 9 . 62 mmol ) dissolved in 20 ml of dimethylformamide ( dmf ) and cooled to - 15 ° c ., was added . the mixture was stirred in an ice - methanol bath and slowly warmed to room temperature overnight . the next day , the mixture was evaporated to an oil which was taken up in etoac , and washed twice with 10 % citric acid , once with h 2 o , twice with saturated nahco 3 , and once with brine . the organic layer was dried over mgso 4 and the solvent evaporated . the residue was dissolved in 100 ml ch 2 cl 2 and precipitated in ether . the product was filtered , dried under reduced pressure over p 2 o 5 and naoh to give a 47 % yield . the material showed a single spot on tlc with r f 1 0 . 57 . 10 equivalents of lawesson &# 39 ; s reagent ( 2 , 4 - bis -( 4 - methoxphenyl )- 2 , 4 - dithiooxo - 1 , 3 , 2 , 4 - dithiadiphosphetan available from fluka ) ( 2 . 8 g , 7 . 0 mmol ) was added to cbz - arg ( mtr )- aie dissolved in 10 ml of benzene , and the mixture was refluxed for 10 hrs . the reaction was followed by the appearance of a slightly higher r f 1 material on tlc . at the completion of thionation , excess lawesson &# 39 ; s reagent was filtered off , solvent evaporated , and the product purified on a silica gel column . the byproduct of lawesson &# 39 ; s reagent was eluted with 100 % chcl 3 , and then the desired product was eluted with 1 % methanol / chcl 3 . after evaporation to a small volume the desired product was precipitated in ether , filtered , and dried under reduced pressure over p 2 o 5 and naoh . tlc showed a single spot with r f 1 0 . 61 . ( the corresponding oxyamide showed r f 1 0 . 57 ). yield : 39 %. removal of both cbz and mtr from cbz - arg [ cs ]( mtr )- aie was accomplished simultaneously by treatment with 31 % hbr / acetic acid for 3 hrs . at room temperature . the product was precipitated in ether and purified on a sephadex lh - 20 column with 100 % methanol as eluent . cbz - lys ( boc )- aie was obtained by the mixed anhydride coupling method as described above for cbz - arg ( mtr )- aie . here &# 34 ; boc &# 34 ; refers to t - butoxycarbonyl , which is employed as a blocking group for the epsilon amine of lysine . cbz - lys ( boc )- aie ( 1 g , 1 . 75 mmol ) was refluxed in 10 ml benzene with 5 equivalents ( 1 . 77 g , 4 . 38 mmol ) of lawesson &# 39 ; s reagent for 10 hrs . upon completion of the thionation , the same workup was followed as with cbz - argψ [ cs ]( mtr )- aie , except that the product was eluted with 0 . 5 % methanol / chcl 3 from a silica gel column . the homogeneous material showed an r f 1 0 . 89 . ( the corresponding oxyamide : r f 1 0 . 83 ) cbz - lys ( boc ) ψ [ cs ]- aie was dissolved in ch 2 cl 2 and was treated with an equal volume of saturated hcl / dioxane at 0 ° c . for 1 hr . the product was then precipitated in ether , filtered , and dried under reduced pressure over p 2 o 5 and naoh . tlc showed a single spot with r f 2 0 . 56 . ( the corresponding oxyamide : r f 2 0 . 48 ). the trichlorophenyl ester of benzyloxycarbonylphenylalanine ( cbz - phe - otcp ) ( 58 . 8 mg , 0 . 123 mmol , 1 . 5 eq ) was combined with argψ [ cs ] aie . 2hbr ( 50 . 0 mg , 0 . 082 mmol ) in 3 ml dmf . while stirring , n - methylmorpholine ( nmm ) ( 22 . 6 microliter , 0 . 205 mmol ) and a catalytic amount of 1 - hydroxybenzotriazole ( hobt ) ( 1 mg ) were added , and the solution was stirred overnight at room temperature . after evaporation of the solvent , the oily material was dissolved in a small amount of meoh / chcl 3 ( 1 : 9 ) and precipitated in ether . the product was then filtered and dried under reduced pressure to give a 94 % yield . the material showed a single spot on tlc , r f 2 0 . 71 ( the corresponding oxyamide : r f 2 0 . 64 ) synthesis proceeds as described in example i , except that afc is substituted for aie to obtain cbz - arg ( mtr )- afc , which is then used in the thionation step . the hydrobromide salt is obtained as described in example ii . synthesis proceeds as described in example i , except that amc is substituted for aie to obtain cbz - arg ( mtr )- amc , which is then used in the thionation step . the hydrobromide salt is obtained as described in example ii . synthesis of cbz - lys ( boc ) ψ [ cs ]- afc and deprotected hydrochloride salt thereof synthesis proceeds as described in example iii , except that afc is substituted for aie to obtain cbz - lys ( boc )- afc , which is then used in the thionation step . the deprotected hydrochloride salt is obtained as described in example iv . synthesis proceeds as described in example iii , except that amc is substituted for aie to obtain cbz - lys ( boc )- amc , which is then used in the thionation step . the deprotected hydrochloride salt is obtained as described in example iv . table i below provides a comparison between hydrolysis rates of trypsin and papain acting on two different oxyamides and on their corresponding thioneamides . trypsin was used at ph 7 . 5 in a tris buffer ( 50 mm ), with 10 mm cacl 2 , and 12 . 5 % dmso . papain was used at ph 6 . 5 in a thiol buffer ( 52 mm nah 2 po 4 , 31 mm dithiothreitol , 2 . 1 mm edta ) and 12 . 5 % dmso . table i lists the catalytic constants k cat ( moles product formed per second per mole enzyme ), k m ( m ), and k cat / k m ( m - 1 sec - 1 ) for the indicated substrates . table i______________________________________substrate trypsin papain______________________________________cbz -- lys -- aie : k . sub . cat 1 . 15 × 10 . sup .- 1 2 . 95 × 10 . sup .- 2k . sub . m 1 . 68 × 10 . sup .- 4 2 . 81 × 10 . sup .- 4k . sub . cat / k . sub . m 6 . 86 × 10 . sup . 2 1 . 05 × 10 . sup . 2cbz -- lysψ [ cs ]-- aie : k . sub . cat 4 . 00 × 10 . sup .- 3k . sub . m no rate 2 . 13 × 10 . sup .- 4k . sub . cat / k . sub . m 1 . 88 × 10 . sup . 1cbz -- phe -- arg -- aie : k . sub . cat 9 . 03 × 10 . sup .- 1 3 . 53 × 10 . sup . 1k . sub . m 5 . 02 × 10 . sup .- 4 8 . 13 × 10 . sup .- 4k . sub . cat / k . sub . m 1 . 80 × 10 . sup . 3 4 . 35 × 10 . sup . 4cbz -- phe -- argψ [ cs ]-- aie : k . sub . cat 5 . 35k . sub . m no rate 5 . 74 × 10 . sup .- 4k . sub . cat / k . sub . m 9 . 32 × 10 . sup .- 2______________________________________ trypsin , a serine protease , showed no measureable rates of product formation for either thioneamide . papain , a cysteine protease , catalysed product formation for both oxyamides and thioneamides . table ii below shows the selective detection of papain in the presence of trypsin using cbz - lysψ [ cs ]- aie . a thiol buffer at ph 6 . 5 was used ( 52 mm nah 2 po 4 , 31 mm dithiothreitol , 2 . 1 mm edta ) with 2 . 5 % dmso . the enzyme concentrations in the assays were 6 . 85 × 10 - 6 m for papain , and 1 . 28 × 10 - 6 m for trypsin . substrate concentrations were 2 . 46 × 10 - 4 m for cbz - lys - aie , and 2 . 53 × 10 - 4 m for cbz - lysψ [ cs ]- aie . rates are given in terms of moles aie formed per minute . table ii______________________________________enzyme cbz -- lys -- aie cbz -- lysψ [ cs ]-- aie______________________________________papain 6 . 03 1 . 15trypsin 1 . 54 0 . 00papain and trypsin 8 . 15 1 . 12______________________________________ the foregoing disclosure of preferred embodiments of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and obviously many modifications and variations are possible in light of the above teaching . the embodiments were chosen and described in order to best explain the principles of the invention and its practical application , to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated . it is intended that the scope of the invention be defined by the claims appended hereto .