永井 良三

心筋の構造蛋白であるミオシンのサブユニットの一つ,軽鎖Iをヒト心筋より抽出精製し,ラジオイムノアッセイによる軽鎖Iの血中での測定法を開発し,さらに急性心筋硬塞発症後の血中軽鎖I値の変動を検討した.抗軽鎖I抗体は,モルモットを精製した軽鎖Iにて免疫することにより作製した.軽鎖IはクロラミンT法により¹²⁵Iで標識し, B/F分離は二抗体法を用いた.アッセイの感度は0.2ng,骨格筋軽鎖との交叉反応は10.2%,成人健常者30名の血中軽鎖I値は3.7±0.9ng/ml(平均±標準偏差)であつた. 24例の急性心筋硬塞症において軽鎖Iは発作後4～16時間以内に上昇をはじめ, 30～144時間後に最高値となり(35.0±14.1ng/ml), 7～16日間高値を続け,各種の心筋酵素やミオグロビンとは全く異なる特異な経時的変動を示した.また前壁硬塞例の中で,異常Q波がV₁-V₃に出現した3例と, IおよびV₁-V₅に出現した5例を比較すると,後者において血中軽鎖Iの最高値, 24時間値, 7日値は高値を示した.血中軽鎖Iの測定は心筋硬塞の急性期のみでなく,発作後時間の経過した症例においても診断に有用であり,またこれにより硬塞の大きさを推定できる可能姓があると考えられる.

ATPase activity and synthesis of atrial and ventricular myosin were compared in the dog and rabbit heart. Atrial myosin showed enzymatic properties characterized by high Ca2+- and Mg2+-activated ATPase activities, low activation energy, lower rate of inactivation at alkaline pH, and no activation by N-ethylmalemide compared with the same properties in ventricular myosin. These findings suggest a difference in the myosin molecule at or near the active site involfing the SH-1 thiols. Also, actin activation of ATPase activities was studied to determine the physiological significance of this observation, since, in living muscle cell, MgATP is hydrolyzed by myosin under the activating effect of actin. Actin extracted from skeletal muscle was used in this experiment. Vmax for the actin-activated Mg2+-ATPase of atrial myosin was about twice that of ventricular myosin, whereas no significant difference was observed in the apparent dissociation constant for actin. This result suggests that the change in the enzymatic properties of myosin is reflected in the contractility of atrial and ventricular muscles. However, [3H]leucine incorporation into the myosin was approximately the same, suggesting that the difference in work load between atrium and ventricle is not associated with alteration of synthesis of cardiac myosin.

A sensitive radioimmunoassay for cardiac myosin light chain II (LCII) was developed, and changes of serum LCII levels were studied in experimental myocardial infarction in dogs. This radioimmunoassay employed an antiserum which was prepared in rabbits against canine LCII. In our assay, 0.2-5.0 ng of LCII were effectively measurable. In normal dogs, LCII concentration in serum was less than 20 ng/ml. The serum LCII level began to rise within 6 hr after ligation of the left anterior descending coronary artery, reaching maximum level at 3-5 days (40-320 ng/ml). In eight out of ten cases with coronary occlusion, LCII could be detected as long as 7 days after operation. In one sham-operated dog, LCII was detected at 2 and 3 days, but its concentrations were less than 30 ng/ml. When LCII was injected intravenously, it dissipated from the blood stream within 48 hr. The time course curves of serum LCII level had two characteristics that had not been observed in serum enzyme studies: 1) LCII level rose rapidly and stayed up during a long period after coronary occlusion, and 2) changes of serum LCII levels were biphasic in six out of ten dogs with coronary occlusion. These results, and our previous studies of synthesis rate of light chains, suggest that when a coronary artery is occluded LCII may first be released from a pool of uncombined LCII in myocardial cells, and then continuously liberated from cardiac myosin molecules. This radioimmunoassay can be expected to be useful when applied to clinical use.