Fish Physiology and Biochemistry 26: 289–295, 2002. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 289 Biochemical characterization of Siberian sturgeon Acipenser baeri and sterlet Acipenser ruthenus milt plasma and spermatozoa B. Piros 1 , J. Glogowski 1 , R. Kolman 2 , A. Rzemieniecki 3 , J. Domagala 3 , ´ A. Horv´ ath 4 , B. Urbanyi 4 and A. Ciereszko 5,∗ 1 Molecular Andrology Group, Institute of Animal Reproduction and Food Research, 10- 747 Olsztyn, Tuwima 10, Poland; 2 Inland Fisheries Institute, 10-719 Olsztyn, Oczapowskiego 10, Poland; 3 Department of General Zoology, Szczecin University, Poland; 4 Szent Istv` an Uviversity, Faculty of Agricultural and Envitonmental Sciences, Department of Fish Culture, Gödöllõ, Hungary; 5 Semen Biology Group, Institute of Animal Reproduction and Food Research, 10- 747 Olsztyn, Tuwima 10, Poland; ∗ Author for correspondence (Fax: (48) 89 535 7421; E-mail: acieresz@pan.olsztyn.pl) Accepted: July 31, 2003 Key words: milt, seminal plasma, Siberian sturgeon, spermatozoa, sterlet Abstract This paper provides data concerning biochemical composition of milt of two sturgeon species, Siberian sturgeon bred in aquaculture facility of Inland Fisheries Institute in North Poland and sterlet (from two different populations from Danube and Odra). Milt plasma of Siberian sturgeon and sterlet, when compared to teleost fish, is character- ized by much lower osmolality (up 70 to 96 mOsm kg −1 ) and protein concentration (0.24–0.58 g l −1 ). In spite of the presence of an acrosome and acrosin the antiproteinase activity of seminal plasma was low (12.79–25.40 U l −1 ). Activities of arylsulfatase and β -N-acetylglucosaminidase were found in spermatozoa. This agrees with the pres- ence of an acrosome in sturgeons sperm. Similarly to mammals, these enzymes are also present in milt plasma. We determined a range of enzymatic activities from the minimal (seminal plasma) to the maximal damage (supernatants obtained after freezing-thawing without cryoprotectant). Activities of arylsulfatase, β -N-acetylglucosaminidase, lactic dehydrogenase and acid phosphatase were released from spermatozoa after freezing-thawing. For this reason they are good potential candidates as a markers of cryoinjury to sperm acrosome and midpiece. Introduction Sturgeon spermatozoa are characterized by the pres- ence of the acrosome which is not present on sperma- tozoa of teleost fish (Jamieson 1991). This structure is well characterized in mammals, contains unique hy- drolytic enzymes and is designed for the penetration of the egg’s envelope (Harrison 1983). The presence of the active acrosome in sturgeon is puzzling due to occurrence of multiple micropyles in eggs (Jam- ieson 1991). Cherr and Clark (1986) postulated that the acrosome reaction in sturgeon has a role as a species-specific block to fertilization, and is not in- volved in penetration through the vestment of egg. Data concerning biochemical characteristics of stur- geon acrosome are fragmentary and were focused on studies of acrosin-like activity of spermatozoa (Ci- ereszko et al. 1994, 1996, 2000). It is not known, however if other acrosomal enzymes, arylsulfatase, β -N-acetylglucosaminidase are present in sturgeon sperm, as well. Our results (Ciereszko et al. 1996) and unpublished observations concerning cryopreservation of sturgeon semen revealed, that after freezing-thawing the fertil- izing ability of spermatozoa was lost. This suggests that some structures necessary for successful fertil- ization were damaged. The extent of cryoinjuries to sperm cells may by assessed by monitoring pro- teins released from spermatozoa. Better knowledge concerning enzyme activities of milt plasma and sper- matozoa are necessary, before selection of proteins