MNC Financial Inc., the region's biggest banking company, said yesterday it has agreed to sell its leasing division and four banks in Colorado, transactions that will enable it to reduce its outstanding borrowings by $300 million.

For MNC, parent of American Security Bank and Maryland National Bank, the deals are a sign that the banking company is moving to raise the cash that it needs to survive the troubles in the region's banking and real estate industries. MNC will use about $170 million of the funds to pay off notes that come due next Tuesday. Some of the rest will be used to reduce the money it owes Morgan Guaranty Trust Co. of New York.

MNC said it is selling the leasing subsidiary to GE Capital Corp., a financial services subsidiary of General Electric Co., and the four banks in Colorado to a subsidiary of Great Western Financial Corp.

MNC said it plans to complete the sale of the leasing subsidiary to GE before the end of the year and the sale of the four banks, which is subject to regulatory approval, in the first quarter of 1991.

In addition to those deals, MNC is considering plans to raise even more cash. Earlier this week, facing difficulty selling its valuable credit card subsidiary, MNC said it was considering selling stock in that operation to the public.

MNC has been struggling with a large portfolio of souring commercial real estate loans, which so far have resulted in losses to the bank of more than $200 million.

These losses have depleted its capital, the money that banks are required to have in reserve to provide a cushion against losses and to help prevent a failure that would require intervention by the federal agency that insures bank deposits.

MNC's efforts to raise cash are aimed at giving it enough capital to meet federal requirements and enough cash to meet its debt obligations.

Meanwhile, MNC's senior credit rating was lowered by Standard & Poor's Corp. The rating agency cited MNC's "heavy concentration in construction and commercial real estate loans."

S&P indicated it will continue to review the debt for possible further downgrading.

" ... With several deadlines rapidly approaching, it is imperative that the company maintain its momentum of quickly selling assets at a time when market conditions are less than favorable," S&P said.

"Until these assets are sold and sufficient cash reserves are created at the holding company to meet these maturities, the ratings will remain on creditwatch with negative implications."