What enzyme leads to the secretion of aldosterone?

Aldosterone is a steroid hormone that's synthesized by the adrenal gland.
It acts on the kidneys to increase sodium reabsorption.
Having more aldosterone around means that more sodium ultimately winds up in the blood, activating thirst mechanisms that cause you to drink water, which increases ECF volume and therefore increases blood pressure.

You might be interested to know that the body really does use aldosterone to regulate blood pressure.
When blood pressure drops, the kidneys sense a subtle change in blood flow and start secreting an enzyme called renin.
Renin works through a pathway called the renin-angiotensin-aldosterone system to ultimately increase aldosterone secretion in order to return blood pressure to normal.

I didn't think an enzyme releases the hormone aldosterone.

It is secreted by the adrenal cortex when sodium levels are low so that the kidneys will reabsorb sodium.
The stimulus would be low sodium levels that had been detected via chemoreceptors.