Науково-технічний бюлетень ІТ НААН -№129 4 DOI 10.32900/2312-8402-2023-129-4-15 UDC 636.12: 577.112.4 PHOTOPERIOD-DEPENDENT ALTERATIONS IN OXIDATIVELY MODIFIED PROTEINS IN THE PLASMA OF SHETLAND PONY MARES AND STALLIONS INVOLVED IN RECREATIONAL HORSEBACK RIDING Kurhaluk N., Doctor of Biological Sci., https://orcid.org/0000-0002-4669-1092 Tkachenko H., Doctor of Biological Sci., https://orcid.org/0000-0003-3951-9005 Institute of Biology and Earth Sciences, Pomeranian University in Słupsk, Poland Tkachova I., Doctor of Agrarian Sciences, Senior Researcher https://orcid.org/0000-0002-4235-7257 Institute of Animal Science NAAS of Ukraine Lukash O., Doctor of Biological Sci., https://orcid.org/0000-0003-2702-6430 T. G. Shevchenko National University “Chernihiv Collegium”, Chernihiv, Ukraine This study focuses on the photoperiod-induced variability in the levels of oxida- tively modified proteins in the plasma of Shetland pony mares and stallions before and after exercise. We have analyzed the effect of photoperiods and exercise on the levels of aldehydic (AD) and ketonic (KD) derivatives of oxidatively modified proteins (OMP) in the blood of Shetland pony mares and stallions involved in recreational horseback rid- ing in the central Pomeranian region (Pomeranian Voivodeship, northern part of Po- land). Twenty-one healthy adult Shetland ponies (11 mares and 10 stallions) aged 6.5 ± 1.4 years old were used in this study. All horses participated in recreational horseback riding. Training started at 10:00 AM, lasted 1 hour, and consisted of a ride of cross country by walking (5 min), trotting (15 min), walking (10 min), trotting (10 min), walk- ing (5 min), galloping (5 min), and walking (10 min). Blood was drawn from the jugular veins of the animals in the morning, 90 minutes after feeding, while the horses were in the stables (between 8:30 and 10 AM), and immediately after the exercise test (between 11 AM and 12 AM). Blood samples were taken once per season for one year: summer and winter. The level of oxidatively modified proteins (OMP) was evaluated by the con- tent of protein carbonyl derivatives in the reaction with 2,4-dinitro-phenylhydrazine (DNFH). There was a statistically significant reduction in the levels of aldehydic deriv- atives of OMP in the plasma of ponies during the winter photoperiods only after exer- cise in both sexes. A decrease in the levels of ketonic derivatives of OMP in the summer photoperiod was observed. These changes were observed independently of the sex and only after exercise. Levels of aldehydic and ketonic derivatives of OMP varied depend- ing on the photoperiod and exercise session in our studies. These changes were depend- ent on the baseline levels of the enzymatic and non-enzymatic antioxidant defense sys- tems in the ponies, which differed between the mares and the stallions (statistically sig- nificant differences in the winter period) both before and after exercise (winter). Keywords: oxidatively modified proteins, plasma, exercise, seasonal altera- tions, photoperiods, Shetland ponies, mares and stallions. The most important environmental factor is the duration of daylight hours, which has a strong impact on physiological processes [39]. According to modern con- cepts, melatonin in photoperiodic animals is a powerful regulator of the activity of the endocrine system and the seasonal rhythm of many processes [17]. Animals make use of changes in photoperiod to adapt their physiology to the forthcoming breeding season [15]. The pineal gland is a neurochemical transducer that converts nerve signals of light