# The effects of the heavy metals cadmium and lead on six metabolic and immune-related enzymes in the loach (Misgurnus anguillicaudatus)

## TL;DR

This study examines how cadmium and lead affect enzyme activities in loach organs, revealing that cadmium has a broader toxic impact than lead.

## Contribution

The study provides new insights into the differential toxic effects of cadmium and lead on metabolic and immune enzymes in loach.

## Key findings

- Cadmium exposure significantly inhibited enzyme activities across more organs than lead.
- Both metals disrupted blood circulation, detoxification, respiration, and nutrient digestion in loach.
- Enzyme activities showed distinct tissue-specific distributions in loach visceral organs.

## Abstract

Cadmium (Cd2+) and lead (Pb2+) are two common heavy metal pollutants in rivers and lakes that have multiple toxic effects on organisms. However, their toxic mechanisms are not fully understood. The loach (Misgurnus anguillicaudatus), belonging to the genus Misgurnus (Cypriniformes, Cobitidae), is an important benthic fish species whose physiological activities are highly susceptible to heavy metal pollutants. Such exposure can adversely affect its growth, development, and reproduction, leading to disease occurrence and significant economic losses in the M. anguillicaudatus farming industry.

After M. anguillicaudatus was exposed to Cd2+ (3.625 mg L−1, 7.25 mg L−1, and 14.5 mg L−1) and Pb2+ (47.5 mg L−1, 95 mg L−1, and 190 mg L−1) for 96 h, frozen sections of their visceral organs (heart, hepatopancreas, gills, kidneys, stomach, and intestine) were prepared. The activities of six metabolic and immune-related enzymes in these organs were assessed using enzyme histochemical staining and optical density quantitative analysis technology.

The distribution of the six metabolic and immune-related enzymes exhibited significant tissue specificity. Acid phosphatase (ACP) was mainly distributed in the stomach, intestine, and gills; alkaline phosphatase (ALP) exhibited high activity in the stomach and intestine; and adenosine triphosphatase (ATPase) displayed greater activity in the heart, hepatopancreas, and stomach. In addition, non-specific esterase (NSE) was abundant in the hepatopancreas, stomach, gills, and kidney; peroxidase (POX) activity was prominent in the kidney, gills, and heart; and succinate dehydrogenase (SDH) activity was enriched in the heart and hepatopancreas. Exposure to Cd2 + markedly inhibited ACP activity in all six organs, significantly inhibiting ALP activity in the hepatopancreas, gills, and stomach, ATPase activity in all six organs, NSE activity in the hepatopancreas and stomach, POX activity in the heart, gills, and kidney, and SDH activity in the heart, hepatopancreas, and stomach. Similarly, Pb2 + exposure significantly inhibited ACP and ATPase activities in all organs except the kidney and stomach, the ALP and NSE activities of the hepatopancreas, gills, and stomach, the POX activities of the heart, gills, and kidney, and the SDH activities of the heart, hepatopancreas, and stomach.

Compared with Pb2+, Cd2+ exerted a broader toxic effect across the six organs. Both heavy metal ions disrupted the blood circulation of M. anguillicaudatus by inhibiting enzymatic activity, impairing detoxification and respiration, and reducing the digestion and absorption of nutrients.