Effects of acclimation and assay temperature on outer- and inner-ring thyroxine and 3,5,3′-triiodo-L-thyronine deiodination by liver microsomes of rainbow trout,Oncorhynchus mykiss

✍ Scribed by Johnston, C. E. ;Eales, J. G.

• Publisher: John Wiley and Sons
• Year: 1995
• Tongue: English
• Weight: 811 KB
• Volume: 272
• Category: Article
• ISSN: 0022-104X
• DOI: 10.1002/jez.1402720604

✦ Synopsis

Abstract

We studied effects of temperature on the deiodinating enzymatic pathways governing hepatic production of 3,5,3′‐triiodo‐L‐thyronine (T~3~) from L‐thyroxine (T~4~) in rainbow trout. Kinetics of outer‐ring deiodination (ORD) and inner‐ring deiodination (IRD) of T~4~ and T~3~ were measured in vitro at 4°, 12°, and 18°C using liver microsomes of trout that were acclimated for 21 days at 4°, 13°, and 18°C and fed on temperature‐compensated food rations. For all temperatures the V~max~ (level of functional enzyme) and V~max~/K~m~ (index of enzyme efficiency) values for the T~3~‐generating T~4~ORD pathway greatly exceeded values for the T~3~‐degrading T~3~IRD pathway; T~4~IRD and T~3~ORD pathways were negligible. The main effects of temperature occurred between 4°C and 12°/13°C; T~4~ORD V~max~/K~m~ rose from 0.48 at 4°C to 1.62 at 12°/13°C and to 1.83 at 18°C. This response correlated with in vivo deiodination rates measured previously. T~4~ORD apparent K~m~ did not change over the temperature acclimation range, suggesting an absence of temperature‐induced isozymes. However, K~m~ was depressed at the assay temperature of 4°C, indicating a direct effect of temperature to enhance enzyme‐T~4~ substrate affinity. T~4~ORD V~max~ was insensitive to assay temperature but was depressed in trout acclimated at 4°C. We conclude that from 4° to 18°C T~4~ORD is the main thyroid hormone deiodinating pathway in trout liver. T~4~ORD activity increases more than threefold from 4° to 13°C. This is due to a direct effect of temperature to decrease K~m~ and to an acclimatory increase in V~max~. © 1995 Wiley‐Liss, Inc.