Laxson Carrie J, Condon Nicole E, Drazen Jeffrey C, Yancey Paul H
Biology Department, Whitman College, Walla Walla, Washington 99362, USA.
Physiol Biochem Zool. 2011 Sep-Oct;84(5):494-505. doi: 10.1086/661774.
In marine osmoconformers, cells use organic osmolytes to maintain osmotic balance with seawater. High levels of urea are utilized in chondrichthyans (sharks, rays, skates, and chimaeras) for this purpose. Because of urea's perturbing nature, cells also accumulate counteracting methylamines, such as trimethylamine N-oxide (TMAO), at about a 2∶1 urea∶methylamine ratio, the most thermodynamically favorable mixture for protein stabilization, in shallow species. However, previous work on deep-sea teleosts (15 species) and chondrichthyans (three species) found an increase in muscle TMAO content and a decrease in urea content in chondrichthyans with depth. We hypothesized that TMAO counteracts protein destabilization resulting from hydrostatic pressure, as is demonstrated in vitro. Chondrichthyans are almost absent below 3,000 m, and we hypothesized that a limitation in urea excretion and/or TMAO retention might play a role. To test this, we measured the content of major organic osmolytes in white muscle of 13 chondrichthyan species caught with along-contour trawls at depths of 50-3,000 m; the deepest species caught was from 2,165 m. Urea and TMAO contents changed significantly with depth, with urea∶TMAO declining from 2.96 in the shallowest (50-90 m) groups to 0.67 in the deepest (1,911-2,165 m) groups. Urea content was 291-371 mmol/kg in the shallowest group and 170-189 mmol/kg in the deepest group, declining linearly with depth and showing no plateau. TMAO content was 85-168 mmol/kg in the shallowest group and 250-289 mmol/kg in the deepest groups. With data from a previous study for a skate at 2,850 m included, a second-order polynomial fit suggested a plateau at the greatest depths. When data for skates (Rajidae) were analyzed separately, a sigmoidal fit was suggested. Thus, the deepest chondrichthyans may be unable to accumulate sufficient TMAO to counteract pressure; however, deeper-living specimens are needed to fully test this hypothesis.
在海洋渗透压顺应者中，细胞利用有机渗透物来维持与海水的渗透平衡。软骨鱼类（鲨鱼、鳐鱼、魟鱼和银鲛）为此会利用高浓度的尿素。由于尿素具有干扰性，细胞还会积累起起抵消作用的甲胺类物质，比如氧化三甲胺（TMAO），在浅海物种中，尿素与甲胺的比例约为2∶1，这是对蛋白质稳定化而言热力学上最有利的混合比例。然而，此前针对深海硬骨鱼（15个物种）和软骨鱼类（3个物种）开展的研究发现，软骨鱼类的肌肉中，TMAO含量随深度增加，尿素含量随深度降低。我们推测，TMAO可以抵消静水压力导致的蛋白质不稳定，体外实验已证明了这一点。软骨鱼类在3000米以下的深度几乎不存在，我们推测尿素排泄受限和/或TMAO潴留可能起到了一定作用。为了验证这一点，我们测量了用沿等高线拖网在50 - 3000米深度捕获的13种软骨鱼类白肌肉中主要有机渗透物的含量；捕获的最深的物种来自2165米深处。尿素和TMAO含量随深度显著变化，尿素与TMAO的比例从最浅（50 - 90米）组的2.96降至最深（1911 - 2165米）组的0.67。最浅组的尿素含量为291 - 371毫摩尔/千克，最深组为170 - 189毫摩尔/千克，随深度呈线性下降且无平稳期。最浅组的TMAO含量为85 - 168毫摩尔/千克，最深组为250 - 289毫摩尔/千克。纳入此前一项针对2850米深处一条魟鱼的研究数据后，二次多项式拟合表明在最深深度出现平稳期。当单独分析魟科鱼类的数据时，呈现出S形拟合。因此，最深的软骨鱼类可能无法积累足够的TMAO来抵消压力；然而，需要更深水域的标本才能充分验证这一假设。
